Improving skin tone is a common objective in photo editing and retouching. Photoshop offers a simple way to measure and correct the percentages of CMYK values in skin tones into the "normal" color range.
Although there is no perfect set of CMYK recipes for all nationalities under all lighting situations, there are guidelines to follow to make sure the skin tones in your subject are within the range of typical skin color for any particular ethnicity.
Once your image is within the target tonal color norm, you can then adjust-to-taste while considering the lighting circumstances and your own personal preferences.
First, check your settings for the eye dropper tool in Photoshop. Make sure it is NOT set on "Point Sample," as it only evaluates a 1 pixel square. For this technique we want to obtain a 5 X 5 average pixel color sample.
Shift-Click to Make a Target Area
With the eyedropper tool selected and while holding the SHIFT key, click on an area that is a diffused highlight (a light area of skin that has image detail).
Avoid specular highlights (areas where the light source is reflecting off of the skin, which lack image detail).
Start a curves adjustment layer by either:
From the menu choose
Layer > New Adjustment Layer > Curves
or, simpler still, click on the new fill or adjustment layer icon in the layers menu.
In the layers palet, select CURVES from the drop-down menu of the adjustment layer icon.
While the curves dialogue box is still open, inspect the info pallet: you will now see TWO sets of RGB values. They represent the color values within the target area that you selected by shift-clicking with the eyedropper tool.
The first set of numbers is the current skin color value, and the 2nd set will show you the resulting changes when you adjust the color values using the curves adjustment layer.
Click on the little eyedropper in the info pallet and change it to show CMYK values instead of RGB values, and we are now ready to begin changing the skin tone color.
To begin manipulating the skin tones we inspect the various channels in the curves adjustment layer. Our image in RGB mode, so there are 4 possible curve adjustments. The RGB channel will effects all of the colors globally, either darker or lighter. We can also select the individual channels to make targeted color changes.
Remember the numbers on the right are our "new" values after we have tweaked the values in the curves dialogue box, and the numbers on the left are our original values we are trying to correct.
In average Caucasian skin, the magenta (M) and yellow (Y) color values should be nearly the same, with slightly more yellow than magenta. Cyan (C) should be between 1/5 to 1/3 of the Y and M numbers. Simply multiply C by 3 and 5 to make sure you are within the average range. Caucasians should have 0 black in the highlight areas. Reducing or boosting these numbers will adjust the "saturation levels" to alter tones for fair-skin or a more tanned look.
Adjust the curves in the various channels to get the numbers "in range." Since our sample was a highlight, make your adjustment point in the highlight area (right side) of the curves adjustment dialogue box.
In this example, the only thing really out of line is the cyan is too high: it's 16, and 16 X 3 = 48, which is much higher than the magenta (34) and yellow (36).
NOTE – when you adjust one channel, it also effects the other channels too, so you need a bit of back-and-forth to get it right.
In this example, to get the numbers within the "normal range" the red channel was adjusted up a bit, and the blue channel adjusted down. After the adjustments were made it appeared a little over-saturated, so the global image was lightened slightly lightening the color saturation in the RGB channel.
When making your adjustments in the curves dialogue box, you might find it helpful to think of the channels as: the blue-yellow channel, the green-magenta channel, and the red-cyan channel, as that is what they actually effect.
A fair-skinned pinkish baby could be as light as 15% magenta, 16% yellow. Most Caucasians fall in the range of 5-20% more yellow than magenta. A fair-skinned Caucasian adult could be as low as 20% magenta, 25% yellow. A bronzed Caucasian could be as high as 45% magenta, 62% yellow. It's easy to oversaturate African-American skin, so be careful there. Asian and Hispanic skin will typically have 10-20% higher yellow than magenta.
Following these CMYK guidelines will help to make sure the skin tones in your subject are within the range of typical skin color. From there simply use your best judgement to fine tune the image.
Here are a few examples of some CMYK values for various ethnicities and saturations of skin tone, but remember that these values can vary widely, so take them as a very loose guideline only.
The above skin tone color correction technique is an adaptation of a Lynda.com training video by photographer Chris Orwig. If you take your Photoshop training seriously, I highly recommend Chris's training. His use of adjustment layers is truly amazing, and really helped me hone my photo editing skills.