It appears that what color the globes on the front of the nacelles appeared depended on the exact parts in use and the precise lighting scheme. The Smithsonian Institution's exhibit page about the actual 1960s starship model has 76 photographs of the object and its various parts. Just looking at the first two images, it is clear that hemispheres may look either orange or pale pinkish gray, depending on the circumstances.
It is hard to say for certain, but those look like two different sets of globes, one much redder than the other. (The redder one, in the first image, lacks the striping pattern seen in the lower image.) The online exhibit site also includes an older photo, taken when the model was newly made, which shows yet a different set of hemispheres (with spines!) as well as a different "front disk."
Actually, I suspect that the solid orange globes appearing in the first picture above are actually ersatz, and the the pinkish ones are real. (See below for why.) See here for what they look like disassembled:
And what they look like when lit up:
The lights behind the translucent hemisphere are clearly orange or orange-red.
Drawing all this information together, it appears that the hemispheres were of translucent grayish plastic, with orange lights behind them. When the internal lamps are off, or the model is strongly illuminated, you see the pale gray or pinkish gray hue. On the other hand, when the orange lamps are on, and the ambient lighting is low, the hemisphere glow orange.
Which color scheme is the "real" appearance is then rather ambiguous. The best you could say would that one scheme represents the way the visual design and special effects crew intended the ship to appear. My opinion on this point is that (to the extent they ever thought especially carefully about the question) they probably wanted the glowing orange appearance. And there is one further piece of evidence that points toward this being the intended color scheme; I think the different globes seen in the first picture, from the 1987 display, were actually a different set that were made to replace the originals when the model went on display, so that the hemispheres would still be strongly red-orange, even without the electric lamps needing to be on.