I thinks it's a color between your light version and your dark version :p, if you check all part of the gun sprite you can see 4 different parts, one wood part and 3 different metal part.
For the 3 metals parts we have the barrel (light metal), the magazine (light blue metal) and all the "other part" (more dark metal).
The "more dark part" is a gray between your two version i think, no?