You make your own alpha layer when you paint a skin. It's normal process. The lighter the shade of grey, the more reflective the paint is. You don't want it too reflective and you don't want it too dull. White alpha would be like chrome, black alpha would be completely mat.
A basic approach to take its, start with a middle shade of grey all over and darken the decals so they mimic a sticker and don't reflect as much as paint. Then tweak the shade till you get your desired finish.
