I use hardware store red primer (various brands) and it seems to do the trick - they also have flat black and white and, I suspect, a gray primer (tried gray ages ago and didn't care for it so not up to speed on that color). There is also a range of dead flat camo paints but can't think of the brand at the moment, Ace Hardware carries it, but no gray.
I wonder if you sprayed white and black at exact same time if you could get a gray primer!
(Don't recommend the experiment.)