Meaning of grey and yellow/brown colors of buildings in Google Maps?

Orange buildings are "interesting", according to Google. In my experience with areas that I am familiar with, these are buildings that are "popular", as likely determined by how many people visit, check in there, etc.

Here's a recent blog post from Google with some information on this: https://maps.googleblog.com/2016/07/discover-action-around-you-with-updated.html

We determine “areas of interest” [orange/yellow areas] with an algorithmic process that allows us to highlight the areas with the highest concentration of restaurants, bars and shops. In high-density areas like NYC, we use a human touch to make sure we’re showing the most active areas.

Orange buildings represent areas of interest determined by an algorithm.