The different colors are meant for different types of terrain, like the sand colored ones are for work in desert regions, while those in darker colors are for urban work. The Length of the weapons also has to do with purpose: the long barreled variants are for Marksman work, where range is critical, while the shorter rifles are for close combat work.
Suppressors also tell you that its for use inside buildings, where gunfire could deafen you (and in general, suppressed shots dont give away your position right away), same goes with the optics, even tho, these days, most operators have a long and a short range optic on their weapons, thats wre this flip up sight comes in: the holo sight is for close range, and then you flip up the other sight for medium range.
So each variant has it purpose here.