I know I'm a bear with a rather slow brain..................
Since I just bought two identical guns, one in 9mm and one in .40, I opted for one to be OD and the other black so it was obvious to me which was which. I have also color coded the magazines this weekend to match. (Well, not the whole magazine, but the bottom of the base plate.) That way, just as the RO says, "STAND BY" and I look down and the mags don't match, I can gracefully exit before trying to fire a 9mm in a .40.