Isn't this group selection?

I would prefer to call it kin selection, but some people might call it group selection. It is one of the few kinds of group selection that actually work.

Surely the much more likely explanation is that producing more or better nectar attracts the bee to you over all the other red flowers.

That wasn't part of my scenario, nor (as far as I know) biologically realistic. It is my bright red color that attracts the bee, and in this regard I am competing with my sibs. But the bee has no sense organ that can remotely detect the nectar. It has to actually land and do the pollen transfer bit before it finds out whether the nectar is really there. So, it is important that I don't provide the color before I am ready with nectar and the sexy stuff. Else I have either wasted nectar or pissed off the bee.