Nitre blueing I believe is what you want. You can achieve a number of color hues depending on the temperature of the molten nitre salts. This method gives excellent "peacock blue" or "fire blue" as well as the nice straw colors used on German Luger triggers and other parts, as well as other shades. It's an easy process, no problem at all to do it on your stove top with small parts. Brownells sells nitre salts in small quantities complete with instructions.