Just using Birchwood Caseys Perma Blue after heating the metal in the oven to about 150-175 degrees will give some impressive color. I also used to experiement with tinture of Benzine (spelling?) and it will give some nice colors if used in the same fashion. Real color case hardening is an involved process that starts with submerging the parts to be hardened in a mixture of bone dust, and wood and leather charcoal. The parts are packed in a cast iron container with the parts, being careful that they do not touch each other, or the sides of the container. A layer of sand is added on top of this mixture and a loose fitting lid is attached. The sand will allow gasses to filter through, but doesn't allow oxygen in (I think that's the reasoning anyway). The parts are brought to about 1400-1500 degrees and are left there for a couple of hours. The longer they stay in the furnace, the more carbon from the bone and charcoal is abosorbed by the steel parts. At the end the parts are quickly quenched in a tank of water to which an airline has been installed to create bubbles that add to the mottling appearance of the finished parts. Usually Potassium Nitrate is added to the quench solution. If air hits the parts before they are quenched the color will be ruined. As with any metal finish, good polishing is a must if the color is to be brilliant. A 400 grit hand polish is about right. Their is also a cynanide casehardening that is used, you see it a lot on the colt clones that come from Italy. I don't like the colors that this method produces, however.