re metal coloring in general;
One of, if not THE classic reference on the subject is the 1936 "Firearm Blueing & Browning – by R. H. Angier". If you go you know where, copies are pricey; $60-ish for 151 pages. But the info collected just isn't compiled so neatly elsewhere; the occasional hangup is on less than modern names for some chemicals. Cross references are available separately, and the web, of course.