The NIB M57 I bought '67 ('S' prefix) had it's cylinder turn plum.
Rugers are noted for the color.
Rebluing those older cast steel framed SxS and single shot break open guns will often leave the frame purple.
Lots of Mausers, Springfields and Lee Enfields reblue OK but then can sometimes change over to plum with some time.
There's a mix of case hardened recv's there .
I did a restoration on a Ross 1910 Sporter that had been 'refinished'.
The receiver was a deep plum color from the hot salt reblue. The bbl was a deep blue color when I recv'd it.
Both had been heavily buffed and the bbl had not been taken off of the recv'r for the bluing process. So both parts recv'd the same polishing compound, prep, oil, cleaner, and tank bluing time.

No strict rules,,no simple answer it seems.
Some blame the investment casting process, but forged and machined parts seem to be effected as well.
Poor bluing salts, temp, time in tank, etc.

There are probably thousands of different steel types then add the different heat treatments they can be given.
It's a wonder anything matches coming out of the hot salt blue tank.

Winchester always had a problem matching color when using the Carbona Blue process as well.(Discontinued use in '38/'39 and switched over to DuLite hot salt bluing)
Especially on the Mod42 where the side plate was soft and the frame HT'd... and on the Mod21 where the frame is Ht'd very tough but the trigger plate which was made of the same alloy recv'd no HT'mt at all.

FWIW,,most parts that refuse to blue w/hot salt blue or turn unacceptable colors like this will generally blue just fine if done by Rust Bluing.
Slow Rust works the best on parts that give problems.

Thanks for giving the old thread new life!