3PO may be brass colored in reality, but was he supposed to look that way in the film? They could have timed the colors to make him look more gold, right?
Before the age of digital intermediates, films were not thoroughly color-timed. The analog process was time consuming: you basically have a guy reading a correction list from the filmmakers, then lining up the shots needed to be re-timed and finally sliding in glass color filters in an optical printer. The process was usually reserved for certain shots to correct white balance (think of correcting the green in florescent lighting) or to create a certain mood (think of the use of blue in Manhunter).
When making movies in an all-film workchain, cinematographers want to make sure the look is as in-camera as possible. This is much more economical.
The question we need to ask is not how C3PO looked in real life, but how the filmmakers knew how he would show up on camera. This is why they would film tests for puppets and makeup - to see how their creations would be interpreted by the film stock.
This is why it is important to take filmstock characteristics into account when doing a restoration. People working on film KNEW how things would look on the screen and adjusted the sets, lights and makeup in order to “compensate” (for lack of a better word). If you don’t do this, you’ll end up seeing the heavy application of rouge on princess Leia and C3PO turning almost green.