They vaguely mentioned their reasoning once in the past... From a competitive standpoint they want you to be able to see and instantly recognize what an enemy player is using so you can adjust your tactics. The logic is utter bullshit and honestly the sound of the weapon tells you what someone is using before you see it but once Bungle has made a bad decision they fight ferociously to keep their dicks in the hornets nest no matter how many times they get stung. But that is why they do it. They don't want you to make a gun difficult to recognize by it's coloring. It's also part of why we don't have a matt black finish. It provides numerous competitive tactical advantages and they don't want everyone to just start running solid black weapons and guns.