Case hardening in itself does not impart colors. It is the impurities imparted by "the pack" which results in coloring the metal. Case hardening is done every day to all sorts of tools and parts. To simply case hardened metal, there are various compounds available. None of them imparts a color. Including Kasenit and Cherry Red:
http://www.midwayusa.com/viewProduct/?productNumber=119479http://www.youtube.com/watch?v=9tlsq2ESQz0Early gun makers would heat a part and then quench in linseed oil, which imparted colors, see "The Gunsmith of Williamsburg" with Wallace Gusler.
http://www.armoryhill.com/review_gusler.htmlHere you can see receivers being packed in charcoal at the Marlin factory prior to heating.
The charcoal imparts carbon, ie, acts as the carburizing agent. In adding carbon to iron, we produce steel. This is from Oscar's articles in DGJ showing a scanning electron microscope image of the surface after case hardening with charcoal.
http://www.doublegunshop.com/doublegunjournal_v7i4_9.htmThis surface color is only a few microns deep. It can easily be removed. It happens to most guns over the years just by contact with the hands. Ken Hurst simply uses some toilet bowl cleaner and a scotch-brite pad to achieve a french grey after the part has been case color hardened.
Pete