I disagree with weapon camouflages just being whatever you've already unlocked for your character. Most soldiers who camouflage their weapons in the real world just use rattle can and objects that create a texture (such as mesh for snakeskin or reeds for striping) and call it a day. To fit the game's aesthetic properly, weapon camouflage being added should be unique spray paints that make the weapon look realistically camouflaged and well worn (paint wears and chips away fast with actual use).
Like @pierogipal said, it shouldn't be an exact match for the equipped clothing pattern. Soldiers don't really paint their guns to match a commercial camo pattern.
There should be a number of specific spray paint camo options for weapons in single-color, two-color, and maybe three-color varieties.
Some guns are also manufactured in different colors, so there could be a non-paint coloring option as well. These are always solid colors like black, grey, tan, or green. The SCAR, M-24, and AK series all come in multiple colors, and the US Army is using Mk.18s with both black and tan rails.