If you're prepared to check out a different brand then the Lyra skintone set is hands down the best I've come across. They're the same sort of pencils as Prisma (blendable, you can use them with Gamsol etc) and the set goes from light caucasian through Asian and African skin tones. It's really useful for nature images, too.
The set was originally designed for kids to use so they could do pictures to depict their multicultural class mates so the pencils are chunkier than Prismacolors (easier for little hands to hold).
Dick Blick has them
here - there may well be other US suppliers, I just happened to remember that Blick stocks them.
HTH!
coloring skin tone
