After 37 years in the painting biz, I can say that you all have nailed it. The trends come and go, and nowadays, they come and go in just a couple years, whereas, a decade ago, the trends could go for 5 to 10 years. Remember when everyone wanted to rid themselves of the avocado green, and the 1970's gold? Well, those colors came back a few years ago.
The gray trend is still very strong. Just remember though, gray (like white), entails so many shades and tones that it's virtually hundreds of colors in reality.
As for good neutrals to sell a house, the many khaki colors work well with just about anything (carpet, furnishings, wall decor). Just avoid going so dark that you create the "cave" effect and you end up making your home dark and uninviting. Also, when potential buyers look at a house with lots of dark colors, they envision a lot of work (extra coats of paint) to cover those colors.