Prepare ahead of time zip-lock baggies with shaving cream inside. Have ready blue and yellow finger paint or liquid tempera paint.
Read: Little blue and Little Yellow.
Activity: Add the blue paint and yellow paint to the shaving cream. Zip the bag (I use masking tape in addition!) Then have the children squeeze the baggie and just play with it. It will turn green!
I let them use a paint brush, open the baggie, and then paint a picture on white paper using their new color. They LOVE this. Make sure you're in a good mood and have a lot of patience! It's worth it.