Since the beginning of photography, shooters have used an 18 percent gray card as a brightness gauge for exposure. When television came along, and then color video, the role of the 18 percent gray card became even more important. It boils down to something approximate to this: 18 percent gray is the average color and brightness of everything. If you are, say, the U.S. Navy (or really the navy of any country big enough to float one), it would very much be in your best interest to paint your ships a color that makes something that big and dangerous disappear. You'd probably call it Destroyer Gray.