The problem is that anything you see on your computer screen isn't reliable. You can find any print, any frame, any theater shot on the net it's still worthless and cannot be used as a reference. At one point, anything you see was taken from an old (thus faded or aged in an other way) print or picture, so it can't look as it original looked to begin with. Add the capabilities of the scanner or the white balance settings of the camera to that, and voila, you're nowhere. It's not even too useful for checking the consistency at least, as different color components can age differently; so scenes shot in different color environment may not suffer from the same fading or color shifting. There are so many uncertainty in the process before you have something on the computer screen, that it's almost useless.

Also, it's even debatable what was original; it depended a lot on the projector. One could project it with a little bluish tint, the other a little yellowish. Not to mention if there's a shift in colors, your eyes will automatically adjust to that after a few minutes into the movie, and you won't notice it's shifted anywhere. Your mind will do the auto white balance. It's the consistency that matters more.

I think BTTF is a relatively simple case. It takes place more-or-less in the modern days and in a realistic universe, and there was no tampering with special tinted filters (which I truly hate in the recent movies) as we all remember. So a reasonably neutral if vivid color balance do this movie justice. And I believe the present transfers are quite OK in this way. You will never now and no one can ever prove if there was a tiny difference to the original anyway.