Much of this was learned from Dan Margulis' book, Professional Photoshop
Great prints: Pleasing skin tone
First, the bad news:
99% of all problems with printing photos are related to skin tones. And it's the subject most likely to make photographers weak in the knees.
Now for the good: Unlike expressions and poses, which are a matter of taste, there's a simple way to measure skin tones and feel confident they're safely in range.
Measuring the colors
If using Photoshop, go to Window > Info to bring up the Info window you see to your right. Then, pick the eyedropper tool. If you hover the eyedropper over areas of skin, you'll see the color values in Red Green Blue (monitor display colors) and in Cyan Magenta Yellow blacK (print ink colors).
Stay with us! It's dead simple. Ignore RGB.
90% of all you need to know is that you can never let the yellow % fall below magenta % on anyone's skin unless you're trying to show sunburn.
Your camera may capture images with less yellow than magenta in skin; unfortunately, they won't look pleasing in print if you leave it that way.
Reasonable magenta & yellow values
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.
Asians and hispanics have typically 10-20% higher yellow than magenta.
Too much color
In the days of film, we used saturated films like Fuji Velvia for landscapes but less saturated films for portraits. No one wants their face to go nuclear. If magenta is getting towards 50%, beware of the well-done look.
The surprising power of cyan
On pleasing photos, cyan usually falls between 30% to 50% of the magenta value. Less than 30% of magenta makes sunburn; more than 50% of magenta makes makes them ghostly blue.
These three photos have the same yellow and magenta values; only cyan changes. Cyan is 24% of magenta on the left (slightly low), 40% in the center, and 60% on the right (too high).
Is it easy to fix with Photoshop?
Yes, and here's how:
Go to Image > Adjustments > Levels. A small window will appear like the one at right. Choose the blue channel from the pull-down menu. For input levels, make the center one become 0.90. You'll see your image warm up. (For really red images, you could go to 0.85.) Click OK.
Go to Image > Adjustments > Hue/Saturation. Choose the red channel from the pull-down menu. Shift the slider to -7. Click OK.
That's all there is to it
Should I ask how fancy people do it?
Yes, because it's easy and you'll delight people with your skin tone mastery.
Go to Image > Mode > CMYK Color.
Now go to Image > Adjustments > Curves. The small window you see below will come up. Choose the yellow channel.
Choose a representative spot of skin, such as a forehead. Move the eyedropper to that patch of skin, hold down Ctrl and Shift (Shift-Command on the Mac), and then click your mouse on the skin. The point you see on the line will appear.
(You can configure the eyedropper to sample a 5x5 or 3x3 pixel area to make it representative.)
Even though this shot was taken with a professional camera and good studio lights, with yellow at 18% and magenta at 22%, Kim is bound to think she looks too pale and too pink.
We will type 30% into the place for output. Then we'll switch to the magenta channel and type 24% in the space for output. Finally, we type 8% for cyan output (33% of the magenta value).
How did we choose those values? They're typical for fair-skinned caucasians and not too far from the original shot.
Finally, Image > Mode > RGB Color. Save. That's all there is to it.