They do seem to come in every color....
I know of two kinds of black aphids. The matte black one you have on onion/garlic and glossy black ones I call "black oily aphid" that swarm my cherry trees.
Other colors :
- tomatoes get the brick red ones and green ones. Green ones typically infest peppers.
- brassicas get the waxy gray ones
- milkweeds get the yellow ones with black legs
- peas and favas get... What do the peas get? ... They get black ones but I can't remember which kind.