What I’ve been doing with my Kindergarten students is having them match a square of colored construction paper with an index card which has the color word printed on it, but that’s after they learn the colors and start to sound out words, etc. Also, I put a tape in the listening center that gives directions for coloring in a coloring page, i.e.: color the apple red, (then I pause to give them time to color), color the backpack blue (pause), etc. That’s a good way to assess if they’ve mastered the colors and can follow oral directions. I also like your idea of using the rainbow to incorporate science into learning the colors. I had some cute preschool lessons on prisms and rainbows, which I think I found on perpetualpreschool.com. Anything you find there, you can adjust to the level of your Kindergartners. Have fun!

Surely you learned this at college!