Oh, I doubt that, @DominicX. I don’t think there’s any connection at all to the rainbow. For me, March is also red (different shade), May is rosy pink, and December is forest green, and those don’t correspond to the rainbow. And no day of the week is red. It simply isn’t a logical association of any kind, just an internal perception that seems to have its origin in an external thing but doesn’t.
For me, 3 is about the same color as A, and 5 is dark red. Again, nothing to do with order of occurrence.