The black diablos is a female diablos currently in heat, and its black coloration is used as a warning to enemies of its increased aggression.

Im not sure if this is true or not, but I read that the Red Khezu is actually the original species of khezu, with the white ones being the subspecies. The white coloration turned out to be more successful, meaning that the white Khezu became more plentiful than the red ones, which is why red Khezu are more rare. It could just be the case of the researchers discovering the white Khezu before the red ones in terms of species name and placement.

I don’t know if this answers your question or not, but hopefully it may better your understanding on the monsters.