Here are some ideas I got from the mailring this summer. One color needs to be oil and the other must be colored water. When I introduced this last year, I held each bottle up one at a time, we discussed the 2 colors and predicted what would happen when I shook it up. After I did the lesson with the kids, the bottles became a much loved and favorite part of our Discovery center. Spread out the glue and have some fun.
Children can make secondary colors from the primaries.
Or use clear cups and droppers with paint or food coloring, or paint in baggies to smoosh around to make the secondary colors. One year, my kindergarteners cut out pictures from magazines and used the picture's colors as a rainbow collage. Just off the top.