I use those small, round, stickers called out on
the packaging as "Color Coding Labels."
I just re-did all of my cans and went with one color. Prior to that I had used
one color for handgun, another for rifle & yet another for shotgun.
I use a felt marker to write in the caliber / gauge.
I've found that they will cleanly come off of the plastic ammo cans,
or, if you are so inclined, you could just stick a different one on
top of another if you switch up what's in your cans.
I've also used 2 x 4" address labels that I
printed up on my computer's printer.
I prefer something easily changed since I change up once
in awhile to utilize my "inside can space" to the fullest.