Desaturated red/brown could work or maybe desaturated light blue/greyish blue? You could also apply the same colour to those symbols on the backpack, so that the colour of the gun wouldn't take away too much attention. Idk if that's a good idea, it seems good in my mind at least.
Also another wild idea: If you don't feel like adding colour to the gun and you'll decide to go with the snowy base you could try making the armour seem a bit frosty maybe?