The surface coating film of color case hardening is a complex mix of oxides of the various trace elements in the carbonizing medium and in the quench medium. As noted above, the visible colors are an indication of film thickness, not pigment. The same range of colors can be had by heating a piece of polished steel. If heated slowly, the color of the whole piece will progress from light blue to dark brown as the surface temperature increases and iron oxides form on the surface. If heated rapidly from one point, the colors will spread as bands of color. Note that in this case, the film is iron oxide, not the same complex assortment as in case coloring. Note also, that case colors can be had without actual case hardening. There are as many variations in color mix and pattern as there are variations in carbon packing, quench medium, and temperature and time control.
Since the surface film is thin and loosely bound, it is subject to any mechanical abrasion and/or acid attack. A top film is required to protect it - poly, wax, drying oil, etc. Anything that removes "rust" is very likely to attack CCH; either chemical or mechanical. The only advice I can give is to go slow and be careful. Keep experiments as small and inconspicious as possible.
Best of luck.
DDA
