We had a week of rainbows back in spring and I found this project all over pinterest. We lightened up the colors a bit and set up a table with all the materials needed. I did one demo and then let the kiddos have fun with it however they chose.
Once their printed ranbows were dry, we trimmed the white paper around the color and mounted them on colored construction paper. One of our kiddos cut his into a cloud shape and another wanted two rainbows on their paper. They chose all different colors of the rainbow for their back ground papers. I wish I had pictures of them! I thought I had...drats!