well yes, that's true, but also no.
if the skins had more variety too instead of being mostly blue/red tones (wow we got pink and light blue for vaporwave now...) then skins wouldn't be a factor.
the teams don't use red and blue because it fits on stickers/skins, they use it cause it fits for the genre "e-sport team". it's not "the best" because in reality it's boring and not unique. if you had to choose the best sticker/logo (ignoring prices) out of IBP, Astralis, LGB, Mouse, Gambit, FaZe, Heroic, Reason etc. you're actually choosing the shape/design and not the colour – that means the logo could have come in any other colour as the colour adds almost no value to the designs uniqueness.
that is not what I'd call "the best". except that (like you said) the resulting stickers fit on the [repetetive] red and blue guns.
Edit: i think you changed what the top said? can't remember what it was before. anyway, what i wanted to say is: you are right, the stickers do fit the best on main weapons, but that's only because those don't have a lot of variety