there's really only two great songs about rainbows, and both come from movies. the first one is "Over The Rainbow" and then there's this one.
this is from the Muppet Show and host Debby Harry of Blondie fame sings it. it was great seeing Debby act and have fun with the Muppets back then. I think a few of those band members are actually smoking!
and for fun, here's Debby and the Muppets singing a real Blondie song: