I've been playing around with the whole "rainbow" concept, because
is having a "Rainbow Contest". There are some really wonderful entries already for it. You should go check them out. Absolutely mouth-watering!
I don't know if I'll submit this for the contest, but I thought I'd go ahead and post it anyway, just 'cuz.
Of course, you will recognize the "spiky" flame. Love 'em! The gradient is a basic rainbow one I made in ApoMap because I didn't like any built-ins or other ones I had. This one is pretty simple. Anyway, made in Apo 2.08 3D Hack and only minor postwork in Photoshop. Enjoy!
Full view for detail and download is 1280x960.