Anecdotally, and this might be a poor example because it's an enthusiast car and not something that most people are going to be encountering, I was looking at used Porsche Boxsters earlier this year, and the more interesting colors actually commanded a premium over silver or black cars because they're much harder to find. I do wonder how that translates to something more practical, but I have a hard time believing that it would negatively impact resale value to a huge extent.
Because someone paid more for that in the first place.
Every dealer is going to have a white, black, and silver/grey at 'stock' cost. After that the price goes up.
People by a car in a color because they like that color and are willing to pay more for it. People buy a car in a neutral because they want a car and don't care about the color.
Logic would dictate that the smaller markets will be less liquid and have wider spreads.