DIY Rainbow Petal Cake. Would take a while to do but WOW what a showstopper! Love this idea for a rainbow party. Could do it on a smaller scale if you decorated a few cupcakes this way instead of an entire cake.
Colorful cookies. Use the palette that would work with your theme.
See "Hummingbird Cake with Petal Tutorial" for instructions on how to make the petal design with the frosting