Did the red roses that made the pink ones perhaps spawn from other roses? If that's possible, I read an interesting gene theory a couple of days ago that might explain this. Basically, flowers would transmit their genes to their offspings.
According to the theory, the parents of a particular flower affect what color offsprings said flower can produce. So it's not as simple as red x red = black. Because, depending on the flower's parents, red flowers might have other genes than just ones that cause the red color. Let's say one of your red roses (or maybe both of them) spawned from a red and a white rose. That'd mean they have both a red gene and a white gene. So maybe, when the breeding happened, a white gene activated in one of the red roses, and a red gene activated in the other one. That'd make a pink rose... They could also make black ones though.
I don't know if this theory is true or if this could be the case in this occasion. But it's all I got.
It doesn't really seem that this is what happened here, if the red roses just appeared out of nowhere. Well, I hope my explanation made sense anyway.
