First of all: congrats on your Diamond! At level 29, that is pretty quick. It took me 127 levels before I got one...
Are you sure about that breeding combo? Knightmare (black/pink/purple) and Pegasus (white/green) are not supposed to breed Clown (blue/yellow). The blue part could be a split-off from Knightmare's black, but there's no yellow in there...