His books are enjoyable, but they don't do much to challenge the reader's 'gray matter'. [='intelligence']
His hair is 'graying'. = He has 'graying' hair.
What will you do when you are old and 'gray'?
(..) drinks on you as a practical joke. Really? And what did they say? Well they said . I said . They said, "The guy in the gray flannel suit. " I think I said, "The, uh, "attractive guy in the gray flannel suit?" Mr. Van Orton. I was afraid you weren't going to make it back. (..)1997 The Game