The Science of Color Correction

by | 3 years ago

Color correction is as much science as it is art. A good color-corrected image can serve as the perfect base for further color grading, or it can be a pleasingly natural photograph at which to look. Unfortunately, it’s an often misunderstood aspect of digital imaging, and it can take a lot of time to master. In this article, I’m going to give you some tips and techniques that I use in my day-to-day image processing to edit colors.

What Is Color Correction?

In digital postproduction, color correction refers to the process of changing colors to reflect their neutral states. Generally, you can achieve this by using a color checker or a gray card in one of the photographs from a shoot, and then using that image as a reference. This helps achieve the “purest” version of your photos. Simply put, if you had white paper in front of the camera, it should appear white.

Color correction also deals with color contamination. What happens when the previously mentioned white paper is close to a neon yellow jacket? It picks up the yellow. If we needed that paper to appear white for some reason, we’d correct that in postproduction. Strictly speaking, is it realistic? No, but in this case, it’s still essential, as we’re correcting a color that was contaminated by a neighboring one.

What about skin color? When strobes are used, the angle of incidence and the thickness of our skin (among other factors) influence the color of certain areas. These are also things we need to correct.

Here are the simple guidelines: White areas should appear white; black areas should appear black; and gray areas should appear gray. The emphasis is on “appear,” as human perception is influenced by context. This means that if a neutral gray color were surrounded by warmer tones, it would appear cooler (bluer). To correct this perceptual “coolness,” we’d make it slightly warmer.

Then there’s color grading, which is basically enhancing the look of an image, and shouldn’t be confused with color correction. Normally, color grading comes after correction.

Don’t Grade Your Conversion

Nowadays, most RAW conversion software packages have numerous color-grading tools. These are useful when you want to lock in the look during the conversion. If you were to do that, however, it would limit your ability to achieve a cleaner, more controllable image. It’s better to do the grading to enhance the look of your image inside Photoshop after you’ve color-corrected the image to a degree with which you’re satisfied. For example, if the whites are contaminated and look yellow, and you applied a color grade to that, then it might shift those already yellow whites to look even more unnatural, and that’s not good. I recommend only using the color Temperature and the green–magenta (Tint) white balance sliders from the color-related manipulators during the RAW conversion process. Obviously, this is just general advice; there are more complex images that require more complex solutions, even in the conversion phase.

When it comes to images shot in the studio, the first thing you should use in Adobe Camera Raw is the White Balance tool (I). With the help of this handy tool, you can pick a point on your image that should be neutral gray, and it will set the color Temperature and Tint values accordingly. In case this produces an undesirable shift, you can always refine it using the appropriate sliders, as colors should look correct perceptually and not mathematically.

The same process would be true for out-of-studio images; but this time, the White Balance tool can only help if you had something in the image that should be fairly neutral gray; otherwise, you’d just resort to manually setting the colors using the Temperature and Tint sliders.

Obviously, if a color checker is used during the photo shoot, the RAW conversion would be mathematically more precise, but even that wouldn’t be a magic bullet, so you should still use the following techniques.

Use the Info Panel

The Info Panel is an extremely useful panel in Photoshop, especially when it comes to mapping and matching colors. In my KelbyOne course, Advanced Photoshop: The Psychology and Science Behind Color Grading , I discuss in-depth how to use the Info panel for color correction and grading. I highly recommend you check it out.

Here’s the gist of it: In Photoshop, go to Window>Info to bring up the Info panel. This panel provides you with all the information you’ll need for any of your color-correcting needs. Now, select the Eyedropper tool (I) and hover over any part of your image. Hold down the Shift key and click to place a color sampler. The color data of that very pixel shows up in the Info panel. If you want to change the color mode, just click on the Eyedropper icon with the downward-pointing arrow in the Info panel and select from Grayscale, RGB, Web, HSB, CMYK, and LAB colors. I’ll show you how all of this is useful in the following technique.

Easily Neutralize a Gray Background

In this example, we’re using a simple studio image, but the same principles would apply to any image. Starting with the background can really help, as it’s usually a bigger area of the image. Here, we have a huge gray background and we want to make it a natural-looking neutral gray without complex selections. If we were to use a different image with a different background, our aim would probably be different: making it warmer, cooler, etc.

Step One: Before we create any selections, let’s put down a color sampler. Find an area on the background that’s fairly neutral in tone and, while holding the Shift key, click with the Eyedropper tool. If you have a new indicator on your screen, and it shows up in the Info panel, you’ve succeeded. Set the color mode in the Info panel to RGB Color.

Step Two: For the selection, we don’t need to be extremely precise, so we’ll use the Magic Wand tool (nested under the Quick Selection tool in the Toolbar). For this image, we set the Tolerance in the Options Bar to 16, and ensured that Anti-alias and Contiguous were both on. Start clicking and Shift-clicking on all the parts of the background until it’s all selected. Then, go to Layer>New Adjustment Layer>Curves to create a new Curves adjustment layer that uses the current selection as its layer mask.

Step Three: Next, double-click on the new Curves adjustment layer’s thumbnail in the Layers panel to open its Properties (in case that panel wasn’t already open), and ensure that the layer thumbnail and not the layer mask is selected. Tip: The selection frame in the Layers panel should be around its icon/thumbnail.

Step Four: Now, in the Curves dialog, we’re going to adjust the individual Red, Green, and Blue channels, which can be found at the top of the Properties panel in the Preset drop-down menu. Switch to the Red channel, and then click the little hand icon with the up and down arrows in the Properties panel. Click-and-hold on the image where you put down the color sampler, and drag the cursor up and down until the two numbers next to the R (Red) in the Info panel are the same (or as close as possible). Move on to the other two channels and repeat this process, matching the numbers next to the G for the Green channel and B for the Blue channel. As we’ve mostly neutralized the image in the RAW conversion, this shouldn’t require much change.

Step Five: After this is done, click on the layer mask thumbnail in the Layers panel. Go to Filter>Blur>Gaussian Blur, and blur the mask by 2 or 3 Pixels, depending on the image’s resolution. Click OK.

Step Six: Then, grab the Brush tool (B), set it to a medium soft brush and, with the layer mask of the Curves adjustment layer still active, paint with white around the hair. You only want the very edge of the brush to touch the hair. This will bleed your correction over the fine strands of hair and make the effect unnoticeable. If we were to correct the background’s color using desaturation, the edges of the hair and the fine flyaway hairs would all look colorless. In the image shown here, I Option-clicked (PC: Alt-clicked) the layer mask thumbnail so you can better see the mask.

Quick Way to Unify Skin Color

Now let’s take a look at a quick-and-dirty method for unifying skin color. The reason you need to have uniform skin color throughout your subjects is that inconsistencies may grab the viewers’ attention, distracting them from the focus of the image. This simple method helps you equalize skin color without having to spend a lot of time using other techniques.

Step One: You’ll need to see the entire image so you can take in everything that needs fixing, so press Command-0 (PC: Ctrl-0). As you look at the color of your subject’s skin, decide which color you want to keep and those you’d like to eliminate—or at least, minimize. For Caucasian skin color, I prefer yellows and oranges, rather than reds.

Step Two: Grab the Lasso tool (L) from the Toolbar. We should set a soft Feather in the Options Bar for all of the selections we’re going to make. I generally use a value between 50 and 100 pixels, depending on the size of the area I need to select and the image’s resolution. Then, we’ll create a selection around the reddish part of the forehead because we want to remove the red in that area.

Step Three: When you have your marching ants, go to Layer>New Adjustment Layer>Curves to create a new Curves adjustment layer at the top of the layer stack. This will bring up the Curves Properties panel where the magic happens.

Step Four: Here’s what you need to remember: red/cyan, green/magenta, and blue/yellow. These are the pairs on the curves that you can use to your advantage. When you switch to the Red channel in the Preset drop-down menu (it’s set to RGB by default), it not only affects the red but the cyan as well, as the absence or subtraction of red will give you cyan. It’s the same with the other channels: the Green channel will give you green when pulled up and magenta when pulled down; the Blue channel will give you blue when pulled up and yellow when pulled down. So, with these basic colors, we can get any color we want.

Step Five: Using the Info panel can give you more accurate results when removing reds, but that would slow you down. I recommend that you play with all the channels in the Properties panel. How, you might ask? Simply click on the middle of the curve in each channel that you want to manipulate to add a single point, and pull that point up and down—not worrying about being precise. This will give you amazing speed and will be fairly accurate, too, as skin color already falls around the middle part of the curve.

Step Six: So switch to the Blue channel, and pull down the middle of the curve ever so slightly until the redness is almost gone. Note: Pulling down more will create noticeably undesirable results. Then, pull up the Green channel just a tiny bit to add a bit of green, as we still have some redness. This already looks good, but it almost unnoticeably desaturated the area. So, no matter how strange it sounds, go to the Red channel and pull up the middle of the curve to add back a bit of red—just enough to boost the saturation, but not so much that it makes it red again.

Step Seven: The next parts we’ll correct are her cheeks and nose; this area commonly turns a bit red/magenta in photographs (especially in flash photography). Using the same soft Feather amount with the Lasso tool, select that area, and then create a new Curves adjustment layer.

Step Eight: In the Properties panel, switch to the Blue Channel and take out the red by lowering the middle part of the curve by a small amount. Then, just as before, use the Green channel to add a little green to the area. This time, however, it doesn’t require any added red, so we’re set.

Step Nine: Finally, choose the Lasso tool again, lower the Feather mount to 30, and make a selection around the camera-left ear.

Step 10: Create a Hue/Saturation adjustment layer (Layer>New Adjustment Layer>Hue/Saturation), and simply lower the Saturation until the ear has the same degree of saturation as the rest of the skin.

It’s that simple. With this quick-and-dirty method, not only can you equalize inconsistencies of color in the skin, but you can also use it for clothing, or any other parts of the image. The great thing about this method is that it’s nondestructive, and it doesn’t make anything look unnatural when it’s done right because of the highly feathered selections.

Fixing Spotty Extremities

The above method is perfect for bigger, contiguous areas; but, as a result of multiple factors, our subjects tend to have spotty extremities that are sometimes on the cooler side, so we need to use a different method to fix this. We have the option to manually correct this phenomenon, or we can use the following trick that utilizes the Hue/Saturation adjustment layer’s adjustment sliders.

Step One: Start by zooming in on the area you want to correct, then go to Layer>New Adjustment Layer>Hue/Saturation to create a new Hue/Saturation adjustment layer above the background.

Step Two: In the Properties panel, use the drop-down that defaults to Master, and select Reds. This will allow you to affect only the colors that fall into the red range. (The extremities usually fall into that range.) If you start moving any of the sliders, however, you’ll see that it affects almost everything that’s red in the image. This is because the default range that’s specified by Reds is too wide, so we need to pinpoint the values we want to correct. Start by setting the Hue slider to something extreme, like +156. This will turn the skin cyan.

Step Three: You can use the color bars at the bottom of the Properties panel to limit the range. The four numbers (two on each side) above the bars describe the start and end points and their gradation. You can manipulate them by moving the little white half triangles and notches between the two bars. The half triangle on the left changes the first number (which is 315° by default); the first notch changes the second number, etc. If you move the outer half triangles closer to the inner notches, that will limit the gradation of the range (or the feather, if you will). The inner notches control the “hard stops” of the range, where it starts to gradually feather outward. It’s always a good exercise just to play around with them.

Step Four: For our image, we need to set the first two numbers to 328° and 351°, respectively. The second set (right side), we set to 8° and 27°. This gives a close-to-perfect coverage for everything we need to correct. The fact that we moved the Hue to such an extreme number helped us to better visualize the range, but we don’t need it anymore, so set Hue back to 0.

Step Five: This is where the fun begins. We’ll start shifting the Hue once again to eliminate the redness in the spots. For our image, this number should be around +13; however, the image goes a bit on the grayish side, so we added a bit of Saturation (+20), and the grayness is gone. Unfortunately, we still have a bit of a problem that’s not too difficult to fix: The spots are still somewhat visible, as they’re slightly darker, so we’ll finish our journey by dialing the Lightness slider to around +5.

Step Six: To make sure that only the areas you want are affected, while on the layer mask, go to Image>Adjustments>Invert to make the mask black to hide the effect. Then, grab a soft brush with white as your Foreground color, and paint back the areas where you want the effect to show.

It’s clear how these simple tricks solved our skin color problems without spending unnecessary hours trying to fix them manually. Obviously, this isn’t the be-all and end-all method, but in the right circumstances, it works wonders. After using these methods, if the skin color itself is undesirable, you can shift it a bit by using Hue/Saturation or Curves adjustment layers. The only difference is that this time the colors are unified and can be treated as one.

Fixing Teeth

We’ll be fixing teeth with this next quick color-correction trick, but it can also be applied to the whites of the eyes.

Step One: Grab the Lasso tool, and then determine the amount of feathering your selection will need. This depends on the resolution of the image, desired precision, and the size of the area you’d like to select. For our example, we’re using 5 px. Once you have the Feather set, zoom in, and select the teeth. If it’s not possible to create the selection in one pass, hold down the Shift key and you can add to your current selection.

Step Two: Go to Layer>New Adjustment Layer>Hue/Saturation to create a new adjustment layer above the Background layer that uses the selection as its layer mask. In the Properties panel, go to the same drop-down as we did for the previous section (the one that currently reads Master), and select the Yellows this time. Now, all we have to do is decrease the Saturation to about –75. It’s important we don’t go the full –100, as it will result in an unnatural-looking grayness.

Step Three: We need to decrease the saturation of the Reds as well, so switch to the Reds in the same drop-down menu. Here, we set the Saturation to –40; again, don’t decrease it too much; otherwise, the combination of desaturating the Yellows and Reds will result in the same unnatural look that I just mentioned.

Step Four: We’re almost done, but we need to zoom out and take a look at what we’ve done in the context of the whole image. Often, the teeth will look darker because of the processes behind Photoshop’s color math, and our perception, so we need to correct this.

Jump back into the Properties panel of the Hue/Saturation adjustment layer, and select the Yellows again. We’ll use the Lightness slider to make the teeth look just a little bit lighter. We can go for a higher number here, so let’s dial in +85. With this, we have natural-looking white teeth.

Note: For the whites of the eyes, use the exact same method, but it’s better to do so using a separate Hue/Saturation adjustment layer, as the eyes will most likely need different values.

Dive Deeper

All of the above techniques can be used in combination with one another. Readers who are familiar with my work know that I like to dive in and manually tweak everything, but in this article, I thought I’d show you quicker techniques to give you something that you can use today. As I mentioned earlier, if you want to learn more about color manipulation and theory, check out my KelbyOne class Advanced Photoshop: The Psychology and Science Behind Color Grading .