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'?
(..) Wow! OK, I'll see you there You're going with him? Don't wear that shirt No, blue Let me look at the gray Not bad, right? Looks nice So help me pick a tie OK You brought all of them (..)1987 Broadcast News
(..) I spilled rum on my jacket Help me pick something out OK I'm thinking the gray suit Tom, why don't I meet you there? I got some stuff to take care of Did you rent the tux (..)1987 Broadcast News