I have never made a sci-fi or castle movie, I like it because it helps tone down on the one-color sets. Meaning that if you build an outside set and make it all grassy green, the green gets kinda overpowering. But throw in even just a bit of grey, and it tremendously helps to blend the colors; making it easier to look at without getting overpowered by green. Of course if you have some custom trees with brown trunks, that works as well. Or if your Squid, throw in everything and it looks great.
I build mostly outdoor sets, so grey works. But when I build a town, a house, or an inside of a building were grey is the prominent color, then tone down the grey with something else, like tan or brown.
So grey is my favorite color mainly because it adds that realistic feel and look.