I suspect that what you have is some self-polination and self-seeding going on...I think purple is a dominant color in the columbines, so it isn't surprising you don't see any yellow anymore. I'd wager though, that in a couple of years you'll see yellow again, and maybe pink and red....Hybrids do this more than species...it's like a couple where one partner is blue-eyed, and one brown. The offspring will be all brown, but (if incest is allowed), then the next generation may have a blue eyed kid amongst the brown, that is, of course assuming that the original parents have double blue and double brown genes...

Thanks michigoose.