Removing Shine on re-done textures

I've been modding the textures for some of the weapon mods, and after I redo them in CS2 they come out in game looking really shiny. How can I make them more matte and less shiny? I just want a flat color on my textures. Anything I can do in photoshop or to the other texturefiles or something? :D
What I did with my mod is I simply painted alpha layer black. It worked for me but I'm just beginner so maybe you will get something more pro from others.