I'm an artist and have experience mixing paints. Considering the large amount of white, grey would be likely. Especially since he used more than one color. As for the amount of time it took to mix the paint... mixing paint is fun, but to get it evenly mixed is a pain. One streak of unmixed paint could mess up what you were going for. And lastly, depending on who you are, greys are very interesting shades :) it's a color after all, and the wrong color can mess up the look or mood of an entire work. Especially with optical illusions. I didnt think greys were interesting till my class did a whole section on how to use greys effectively... my eyes were opened to a whole new world after that lol
