Gray was a color. One shade was iron gray. (TNG: "New Ground")
Hair turning gray was a sign of aging. (ENT: "Fortunate Son"; TOS: "The Man Trap", "The Deadly Years"; TNG: "Man Of The People", "Aquiel"; DS9: "Distant Voices") A tricky ethical area was said to have shades of gray. (DS9: "Hippocratic Oath", "Waltz")
In 2375, to hide their theft of a cloaking device, Quark and Rom pretended to be staring at a bulkhead, claiming that Rom wanted to paint his quarters the same color. Benjamin Sisko pointed out that they were already that color. Quark then reminded Rom that he had told him that very thing, to which Rom replied, "No wonder I like it. It's a very soothing shade of gray." Sisko then answered, "I'm glad you approve." (DS9: "The Emperor's New Cloak")