Boy, I feel really nervous about disagreeing with the moderator, but all my sources say the Mausers were all case hardened (except possibly the late FN's, the Mk. 10's, and anything later.) Modern alloy steels didn't exist at the time--by casehardening what we would call "mild steel", Mauser created a hard 'slick-acting' surface which added a lot of strength, while retaining a soft, tough inner core that wouldn't shatter.
The case hardening was not as perfectly controlled as it would be today, thus some have a much deeper case than others, leading to the drill and tap problems George mentioned. (For the same reason, Rockwell testing of Mauser recievers can give wildly varying results, yet still be safe.)
As George noted, color-case treating amounts to 're-heat-treating', which most authorities consider a no-no, or 'dubious at best'. I saw a color-cased Mauser awhile back in "Rifle" magazine...a fancy number that, knowing the gun-scribes, probably wouldn't get shot much anyway. If I were to color or re-treat one it would be for nothing hotter than a factory-load 7mm Mauser.
The book you really ought to look at is Jerry Kuhnhausen's "The Mauser Bolt Actions-A Shop Manual" (Brownell's and elsewhere). He does mention a possible re-heat-treater for Mauser actions; if my old note is correct, it's 'Metal Treaters Inc.', of St Paul, @612-646-1316...that's an old scribbled note, so you may have to do some searching.
best, weegee