Light interface coloring I know can be done to stainless steel, although I have never seen it done to any
mods. To my knowledge it is similar to anodizing aluminum, it is done using a electrochemical bath. And produces a controlled corrosion to the steel. I could not find to much info via google, but again I have heard of its uses for constructing and architecture purposes. So I do not see why its not applicable here. My advice if you are looking for a decent cosmetic change for your
mod, if you can find a aluminum clone of whatever piece you are trying to color, have that anodized.