Era should have a shop where you give them the rare ores (gold, rubies etc) and be able to trade in a weapon and rare ores and in return recive a changed gun color. But it would varey on how much minerals are nedded. The bigger the gun, the more minerals required. But it wouldnt change the whole gun color. It could also be different patterns, such as striped, wavey, or solid. Example lets use s M4 black is a nice color to use for a example; you can have the option to have the current gun color, black, and have red stripes in the gun. And be able to select the whole gun, or certain parts (butt, body, or barrel). I want the whole gun done, so i would give 3 rubies and Money and get a m4 with red stripes.
The gun colors would be following
Rupies. Red
Sapphire. Blue
Emerald. Green
Gold. Gold (duh)
Diamond. Turqouise
Gypsum. White
Quartz. Brown