I've been on a roll with tutorials lately, so why not another one. This is really simple to do no matter what program you use.

1) Open up whatever graphic you what to do this on. I picked one of my old graphics.

2) Use the dropper tool to pick up the skin color of the person.

3) Using the paint brush, set the hardness to a low level so it doesn't look blocked in. Pick a size that's small enough to cover only the face and nothing else.

Tip: Using the quick selection tool you can select the face making it easier to color in without going outside of the face.

Tip: I would do this on a different layer just incase you make a mistake and need to erase it.

4) This is the finished product. You can then play around with the adjustments.