Every kid loves play dough. I make a few big batches of red, yellow, and blue. Then the kids can mix the colors to make their own favorite colors. We also add essential oils to make it smell great! Here is the recipe I got from my son's Early Childhood class. This is a great activity to pair with a fun children's book on mixing colors.
5 Cups Flour (I go with unbleached whenever possible)
1 Cup Salt
2 Tablespoons Alum
3 or 4 Cups Boiling Water
4 Tablespoons Cooking Oil
Mix Dry Ingredients. Add oil (and food coloring if you want just one color) to boiling water. Knead as it cools.
I personally just boil the water in my tea kettle, then pour it into a glass measuring cup, add the oil and food coloring, then dump everything into my stand mixer. I start with 3 cups of water and add more if needed. Then just use the dough hook to mix it all up. It's much nicer than trying to knead dough made with boiling water. After everything is all mixed up, I add an essential oil to match the color (orange, lime, lemon, peppermint, etc)
The kids absolutely love it!