Updated June 10th, 2019.
Ah, the Mac’s red, yellow, and green buttons: red for “close,” yellow for “minimize,” and green for “maximize” (sort of). You kind of get used to seeing them in that same red-yellow-green order. Until one day they’re all gray, like this:
Yuck. This is not nearly as good as red-yellow-green. If your Mac’s buttons are gray, and you want to bring back the color, just go to…
Apple menu/System Preferences/General
and choose something other than Graphite as the Accent Color. (In Mojave, you get several choices. In other versions of the macOS you see just two: Graphite, and Blue, under the heading “Appearance.” Choose Blue.)