I've been working on uploading fractals from 2002 to my gallery, and finding many that are full of rainbows. That must have been when I first started experimenting with my three-layer technique. I remember being all interested in atmospheric optics around that time, so probably that was what inspired me to try making pictures with that kind of look.
And it also reminded me that I've been meaning to write a page about how to put spectra into fractals, and now I have. I hope somebody out there may find it useful.
Here's a new fractal that I made while I was messing with some of the tutorial images.