It took me a long time to figure out the usefulness of gray, and then more time to figure out how to make it. Gray is useful because it allows the other colors to sing. You make it - well, there are lots of ways, but my way is to mix blue, red and yellow, and generally white. This way you end up getting a large variety of grays, which is more interesting in my opinion. That versus mixing just black and white or burnt umber, ultramarine and white, or gray straight from a tube.