Background

When I attended Joe McNally’s workshop a few years ago in Hong Kong, He said that Light has 3 characters, Intensity (hard light/soft light), Angle and Color (Cool/Warm).

This principle applies when you’re taking photos outdoor with flash/strobes, in which probably you will find a mismatch color issue with the light comes out from your flash against the ambient light which will cause uneven color distribution on the skin tone of your subject.

It happens particularly if the photoshoot takes place in the morning or afternoon when the color temperature of ambient light is warmer than the light comes out from your flash. In this case the color temperature of your flash (~ 5600K) doesn’t match with the available light (between 3000K-4000K) which causes some area on the skin of your subject looks yellowish and the other area is more blueish or magenta (see picture below).

How to fix it in Photoshop

This issue can be avoided in the first place by putting a CTO gel on your flash to match the cooler tone of your flash light with the warmer tone of ambient light. However you can still fix it in Photoshop if you don’t have CTO gel.

As I learned through the internet, there are many different ways to match skin tone in Photoshop. You can adjust the color and/or brightness intensity on the Level, Curve, Selective Color or Hue/Saturation of the adjustment layer.

I will show you one of those approaches which I think pretty simple and straightforward by using the selection color adjustment layer.

1. Open the Info panel (Window > Info) to see the color distribution information of the CMYK colors
2. Click I to activate the Eyedropper Tool and hover it to the area which appears to have the mismatch skin tone issue. In this case, you see the highest percentage on the top picture is Yellow (56%) and the lowest percentage is Cyan (27%). Then compare this condition with the colors distribution on the bottom picture. It appears that the highest percentage goes to Magenta and relatively has even color distribution compared to the top picture. We will use the color information of the bottom picture as the reference information to match the skin tone by nudging down the yellow color. 
3. Open the Selective Color adjustment (Layer > New Layer Adjustment > Selective Color) and it will open with the white mask which means any color adjustment will be seen immediately.
4. Change the Layer Blending mode to Color to preserve the brightness level of the area that you want to make adjustments.
5. To make correction on the skin tone color, you can adjust the colors and brightness on the Red and/or Yellow channels. In this particular case, the objective is to reduce the amount of yellow color. Select the Yellows channel and move the slider of the yellow color to the left until you find the color matches with the rest area of the subject’s hand. This is actually you are adding more blue color on the skin tone as the opposite color of yellow is blue. Double check with the Info Panel to ensure that Yellow color has equal percentage with Magenta. If you see the skin tone is too cool, you can add Red color by reducing the Cyan color (i.e Red is the opposite color of Cyan). You can also increase the brightness by reducing the black color intensity. Please remember the rules of the color channels: Cyan <—> Red; Magenta <—> Green; Yellow <—> Blue
6. Now you invert the white mask of the Selective color adjustment to black mask by pressing Command+I (Mac) or Control+I (Win). Then select soft brush with 20% flow rate and 0% hardness rate. Brush the target area with white color to reveal the previous adjustments on the selective color layer.
   
7. That’s it, you can see now the skin tone looks matching across the subject’s hand. You can create multiple Selective Color adjustment layer if you find mismatch issue on multiple area of your subject.