Well.
My opinion anyway:
Snipers - Yellow
Riots - Red
Enemies - Green
Allies - Blue/Violet
Weapons - Hot Pink.
Method behind my madness? Gotta watch out for snipers (Most sensitive color), Riots don't need to be seen from far away, because they pose no threat (least sensitive color), enemies must be bright/easy to see (sensitive color), allies don't need to be seen as well (blue/violet), and weapons don't need to much attention since the enemies are bright as it is.