I have a huge stockpile of magazines.
Some are 30 round, others are 50, others are 85, 120, 330 or 400+, all of them, for the most part look similar being as it's an M4...I also always carry one or two magazines with different ammo weights in (my "stock" weight is .25, I tend to carry one 85 or 120 rounder of both .2 or .3), so I label my mags using tape to designate their weight and then they have symbols on them to designate capacity (my plastic 85s for example have to triangles, red and blue base to base on each side)...
When I finish my tracer unit I'll also use something to designate tracer magazines and different ammo colours (some colours are more useful than others in certain circumstances)...
Most people just use it to designate mag type or "mag priority" (A friend I have uses a rainbow system to show which mags he should use in which order)...