Does anyone know why Rad is half Red Heeler when BOTH of the Heeler (grand)parents are Blue?

I don't see anything anywhere that says Radley is only a half brother, let alone the fact that I don't think half blue half red heelers even exist? Pretty sure when a red heeler has puppies with a blue heeler, the litter is just mixed with red puppies and blue puppies. Even if I pretend that this isn't the case, it still doesn't make any sense at all because neither of his parents are red heelers.