Originally Posted by ADU
I believe BRFC was transferred from negatives btw, which frequently seems to obfuscate (ie "muck up") the original color of a film. Not sure how Alien was done.
It's not about "mucking up" anything as such, because basically the original negative does not hold a record of the final timing. So, when a movie gets scanned from the neg onto video the colour, contrast etc needs to be redone within that video realm. Ideally they'd use an extant positive element as reference, along with stuff like camera reports so they know exactly how to process the image in certain shots, i.e. if there are any photochemical tricks that need to be replicated, like day-for-night. If the director or DoP is no longer around then that information is vital in maintaining the proper look of the film.
But, as we've seen with Sir Ridley and James Cameron, if the director is still breathing then they'll usually take it upon themselves to adjust the colour, contrast, framing etc for each new release. The mastering idiosyncracies of the period will have a part to play, and Alien is a great example. The 1999 transfer looks different from the 2003 transfer, and the recent 4K redo is different again.