Hmmm. I just happened to be reading the 1999 FSM about that very thing. To quote page FSM 6-832: "The upper and lower injector O-rings use different part numbers. To identify the correct O-ring, the upper O-ring is black and the lower O-ring is brown. Both O-rings are the same size but consist of different material."

I have no experience with them and maybe in practice it makes no difference, but I thought it was worth mentioning.