I've seen guys (like SWAT and anti-terrorist units) with blue or yellow tape on their MP5s and M4s, I believe either to signify training weapons and/or training mags, such as this MP5 pic:
Could also be used to signify the type of ammo as mentioned above, found this pic that has a single mag with blue tape on it that separates it from other mags (could be FMJ, JHP, or even safety rounds (penetrates flesh but breaks up into powder on contact with a hard surface....... to prevent ricochettes and wall penetraion for bystander safety).
And for putting tape on the bottom of the AK mags to keep the springs in, someone should tell this guy about that trick, considering he put the tape on the wrong part of the mag:
