i gotcha, sorry i didn't see your other post until after, but it all deals with what area of the gun you select.
basically:
1.) duplicate base layer, then hide it
2.) select the area of the gun you want to color with the various selection tools
3.)when you have the desired area do ctrl+c, add a new layer (name it "new gun" for purposes of these instructions), and ctrl+v
4.)add a new color layer (name it "new color")
5.)select your new color layer
6.)ctrl+click on the THUMBNAIL image of your "new gun" layer on the layer panel
7.)go to the selection menu at the top and then "invert"
8.)select your "new color" layer and press delete (it should delete the space outside of your new gun shape, so that the color fills in the same space as the new gun layer)
9.) change the "new color" layer blending to multiply, overlay, etc. (any type of layer style you want...perhaps even play with the layer styles, add a gradient overlay, do whatever, go crazay)
you can find free stock photos of textures here:
http://browse.deviantart.com/resources/textures/
just make sure you read and follow the artist's rules