Two of them, actually. During a delightful sun shower.
I've been thinking since then that never mind the colors, the shape had to be really startling to early humans -- so simple a curve, so unlike the cragginess of rocks and trees and clouds. That, perhaps, even looks like it's holding up the sky. So it just occurred to me: I wonder if the original impetus for the arch as a structural element came from marveling at rainbows?