Actually, it can be done. You need pastels, q-tips, a spray sealer, and masking tape (optional). Warning, this isn't a quick process.
Depending on the doll you might need to use a bit of masking tape around the joints before continuing.
1. Spray the desired area with the sealer to give the pastels something to grip.
2. Get a rather nice pile of pastel powder on a sheet of paper until you think you have more than you need. Then add another 1/4th to that. It takes a lot.
3. I tend to use white, but if you prefer a more natural look mix a bit of white with the color of the skintone. Fill up your q-tip and prepare for cramped fingers. Press hard and rub the pastels in. If the vinyl bends you are pressing to hard and need to lighten just until it no longer bends.
4. Brush off any excess pastel powder with a soft paint brush gently.
5. Seal the layer.
Here's where the patience comes in. See the vinyl will only take so much of the pastel at once. Even the sealer before can't change that. You need to layer it to get the desired effect. So repeat steps 3 through 5. For the first couple off layers you won't notice any real change. That's natural, and it is the reason I said to add more pastel powder to your pile. It will take time and a whole lot of patience but it can be done.
I've used the this to change skin color on several dolls. I've made them both darker and paler. I just don't have pictures of the lighter one.
Monster High CAM Vampire boy got natural coloring.
Ooak Monster High CAM Vampire by
faydreams, on Flickr
While a Basics AA Ken went from this:
SAM_2384 by
faydreams, on Flickr
To this.
20120207_2 by
faydreams, on Flickr
That was as dark as I could take him before loosing the details of the sculpt. I think on the two of them there are at least 12 layers of pastel to get the look.