I have been asked this in other forms about skin tone. So I am going to talk a little about a guide that I use. This is something that I use to start with and only a Guide that I follow.
First Let start with a Photograph
Your object here is to get the skin so that it is evenly as possible with out losing to much detail down the road. For when you reach this you will be able to make more adjustment on the photographs.
First thing that you need to do is get a sample of the colors on the skin. To do this you need to use your eye dropper tool that has the cross hair in the corner. Shown here.
Once you have the tool selected, you need to be sure that the tool is set to only sample an area of 5×5 on the skin. But where to select is always a question that is asked. Well when you are choosing an area, you want to look for areas of the skin that do not have any makeup on them, not too bright were the flash has hit directly and the tones are defused. A common area that are bad for this are cheeks. The reason for this is that they normally have makeup on them and you will not get a good tone or sample of color from them. Normal areas that do not have makeup, but may have only foundation, are near the eyebrow and the chin.
At this point the information that you will be getting will be in RGB settings. You need to change the information to CMYK. This will display the information as percentages. To change the values from RGB to CMYK, all you have to do is click on the Eye Drop icon where you see the RBG information and make that adjustment.
One thing I do suggest that you do is make a copy of your back layer. I did not state this earlier, this will not effect the information that is being read by the tool. This will help you down the road if you plain to make any adjustment to the photograph later. So let us return to the topic at hand. You need to add a CURVE ADJUSTMENT layer to the the photograph. For this is where we are going to make the color tone adjustment.
I was using CS3 but the information is still able to be use in any of the Photoshop CS programs. CS4 gives you more options and easier to work with.
Before you can make the adjustment you need know the CMYK values to look for when you are making the alteration to the photographs. A guide line that I have found to work as follows.
C = 1/3 to 1/5 of M and Y
M = Magenta and yellow are close yet
Y = Yellow is higher than Magenta
K = 0
So what you need to do is Divide Yellow or Magenta by 3 and by 5 to gain the value. from there you want to be between these numbers on your adjustment. So with this photograph the adjustment that I need to make for this photograph would be in the rang of 8% to 13% and I would start my adjustment on the red channel until I have reach my value.
Then I would then make adjustment on the other channels so that the CMYK will fall in the guidelines.
To confirm that I have it close to the guidelines I will recheck all of this.
C=13 * 3 = 39 * 5 = 45
C=9 * 3 = 27 * 5 = 45
Then confirming that M and Y fall in this rang and yellow is slightly higher and K = 0
and you will end up with a photograph that is closer to true skin (left) tone then what was before (right)
After you have completed all of this, then you are able to make any other adjustment that you feel is needed.
Keep in mind that this will not always work on every photograph and there are other adjustment that need to be done on different skins. For example, Dark, Tan, Light, and Olive are a few. This guideline is like making a cake, you use the base of the information and then you adjust it to your taste or to the cake you are going to be creating.
With this photograph there are other things that I would need to do in order for me to complete it. But this is only how I start out and then from there I make the needed adjustment to the photograph to get the best results of the person and the surrounding area.