wow - did you see that honey... up in the sky.

I once saw a treble rainbow in the sky. they were a little faint but discernible.
We all know the story of Noah and how ridiculous it is to cram all of creation onto an ark.
I recently watched the Russel Crowe version and that version made it even more incredible
with the stone monsters? So looking at it from an atheist point of view.. shouldn't there be
examples of rainbows in prehistoric art. They have paintings of the sun,moon, deers, animals..
but I couldn't find a cave painting of a rainbow. How hard would it be to paint an arc. One sweep
of the hand. a few colours? the flood was supposedly 2,400 BC.
So is there any examples predating 2400 BC. ?