hey man, what i do is spray the color of paint you want, usually black, or a rusty color, or both, onto the corner of a cloth rag, and then lightly tap of the excess onto another rag so you dont have too much paint, and then lightly rub the remaining amount onto the sticker or decal...make sure you dont have too much on the rag before you apply it...that is the trick...practice on some other decal to get the amount you want...i think it is called "dry brushing" or something...you could even use a brush, dip it into the paint, then take a towel, or rag, and get rid of the excess, then lightly apply what is left in the brush...get into the corners of the skull, or patch, and on the edges, also along the actual edges of the decal, where it meets the armor...basically where grime would accumulate in normal wear circumstances...
D