All my 5.56 mags are from the same mfg. and in basic black.
My .300blk mags are either 'oddball' 5.56 mags or Lancers (translucent FDE) or D&H .300 mags. All the black .300 mags are painted tan and marked '.300bo'. When I painted my D&H mags I masked the '.300' logo with a sticky target dot.
I paint all my 'variant' AR mags with color, tan for .300, red for 5.45x39, green for 7.62x39. Easiest way for me to identify them when fishing for one out of my range bag.
And I still give the top rounds a quick glance before loading...just in case. And I store my caliber specific mags separately as well.
This may be obvious, but I don't paint the part of the mag that seats in the magwell, just what shows. Tons of tips in the refinishing section on how to get crazy with a rattlecan finish, all my rough surplus AK mags sport a different paint scheme now. Just call me 'Michelange-low-and-to-the-left'...