It's one of the sample scenes but I turned up the chromatic dispersion on the thing. So some rainbows come in.

At least more than what it once was showing...

I thought about what it would take to simulate a rainbow.
That would be pretty if I could get it to work. It would take a
lot of spheres of water. I figured I could arrange them in a sheet
and use the sun and a camera to get it. ... We'll see.