Amazingly, there are some folks who say the blue version of Regula is EXACTLY how it looked in the theater 31 years ago!
But I have now read at least 5 different posts from people who have seen vintage prints stating, yes regula is red/brown in vintage 35mm prints!
Trouble with this assessment that when films deteriorate with age they go red (water from JAWS turned pink less than two decades after its release), so if they looked at the prints even 10 years later, they were already probably fading that way. I saw a theatrical screening of a 35mm print of TWOK less than a decade ago and it was so OTT red it was almost like there was a filter over the movie and it most definitely did not recapture the theatrical experience I had with the film (7 or 8 times during first-run, and at four different cinemas.)
TWOK is a very red movie compared to TMP, but that's true even on the BR. I think what most people are remembering of the film is the way TWOK looked on VHS and laserdisc and DVD, which was pronouncedly red, probably from somebody cranking a knob too far.
It's probably hard to get a good line on what the color is supposed to be, because of the ambient color lighting on the bridge scenes and other sets. Part of the problem on these films is that nobody's skin tones look all that good, probably because they're reflecting all these unpleasant grey to beige tones off the set walls onto the faces. I remember thinking that when Kirk squares off against Kruge near the end of SFS, 'this is the first time since the series that anybody has got good skin tone' (probably the only thing I remember liking about the last half of that movie!)
For some reason this issue seemed to be minimized once they got the new bridge in TFF & TUC; even though the walls differ drastically (from ARCHITECTURAL DIGEST warmth to submarine movie grey) in these, you don't get the feeling these people's lifeforce is being sapped up by the bulkheads.