So, there aren't a lot of reasons to change a person's skin color. For realism, it's often easier just to find someone with the tone you're looking for. Unless, of course, you're playing a vampire or a mutant. Vampires are notorious for having very pale skin. From Dracula to Edward, it's the one (and sometimes only) thing they all have in commmon.
So we'll start with that.
Find a good picture of your PB. I'll be going with Cristina Scabbia (the more pale they already are, the less work you have to do, and the better the results).
That's our starting picture. Pretty, no?
Making people pale is pretty easy.
Go to the Layer Palette and click to make a new layer. You can call it whatever you like, but make sure its blend mode is altered to Saturation, and that it's at 100%.
Fill the Saturation layer with pure black or white. It doesn't matter, since this will remove all color.
Two parts to this. Erase any of the saturation layer that isn't on skin.
Then mess with the opacity of the layer until it's the way you like it. If you can't make it white enough for your tastes, you might want to add a very light lighten layer, but be careful, since this will dull the features of the face and is generally a pain.
That's a happy amount of pale for me, but your mileage will, of course, vary.
That wasn't so hard, was it? Now the real fun starts when you want a character who's blue/green/pink, etc.
This one's a little more complicated, but not any harder.
Add two layers to your picture. Label one under, and make its blend mode multiply, and name one over, and make its blend mode hue. You can use color if you prefer, it gives a more vibrant appearance, but I find that hue looks more realistic. Use which ever makes you happy.
Fill both layers with your desired color. I tend to make the multiply layer a little darker than the color I want for my end result.
Erase around the skin for both layers. The easiest way is to give one layer a really low opacity and work on the other layer, then switch. Lowering the opacity on the multiply layer while you're erasing it helps a lot, too.
Blur the multiply layer, especially if you've got jagged edges like the ones around her fingers in the picture. Adjust the opacity on both layers until they're at a level you like. I tend to start both around fifty percent, and work from there.
This is probably a good time to emphasize why working with a big picture is preferable for this sort of thing. I could not get the line on her hand smooth to save my life. Giving yourself the whole picture to work from lets you omit areas that give you difficulty, instead of losing your mind over them.