Imo you should make the stock green or tan to fit with the modern theme of the rest of the pack, extended the barrel abit and adding a proper flash hider would make it alot better too.
And the magazine needs a tad more detail. Just a couple grooves on the sides. And between brown or green, I'd like either one. Not tan so much though.
It wasn't long ago I was talking about how cool an M14 would be. And since the M21 is a sniper adaption of the M14, I think it would be cool to keep the sniper brown as it is. And once it's totally finished, you make an M14 version, which would be the same model copied over, but the scope removed, and the stock turned green.
It wouldn't serve much of a purpose other than being another assault rifle type weapon. But it should be pretty easy to do compared to making the M21 from scratch. And it would be really cool, as I said :P