I feel like such an elementary teacher (which I've never been, except at home), but here's yet another typical teacher post about a good craft product. A teacher at work shared her recipe for homemade play-dough. I made it, and the kids love it. It turns out pale.
I recommend cooking it in a skillet on the stove top on low heat. It thickens quickly; stir it faithfully.
My friend said to use icing color, and it worked well, but it's hard to find in lots of colors (at least at our WalMart).
The icing color is a sticky paste. The second time I made this play-dough, I used regular liquid food coloring in the plastic bottles, and it worked very well too. That gave me yellow and blue too.