They cannot be selected. Gun camo is determined by what gun you pick up. For example if you pickup a SCAR off a cell soldier it has this urban camo, pick it up off a weapon crate it will have the default green paint, and I believe there's a one for the marines that is painted sort of like multicam.
You cannot change it I believe in single player, but someone will probably make a mod for it. You probably will be able to change colors in MP. After all one of the Special Edition's ingame items is another gun camo.