I definitely get the markings for 300BLK and 556 too though that would be another valid reason to colorfill no?
If one were to be "marrying" a Lower to an upper
This is why I rattle canned my 300Blk and several mags....besides the fact that it looks bad ass to me, I know never to put 5.56 rounds in the rattle canned mags and never put the rattle can mags into any of my AR's. I even marked my 5.56 can so I never accidentally put it on my 300Blk....I have two AAC cans; 762-SDN-6 and M42000.....I'd have a really bad day mixing them up.