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'?
(..) How nice. You remembered. That was the day the Germans marched into Paris. Not an easy day to forget. I remember every detail. The Germans wore gray, you wore blue. Yes. I put that dress away. When the Germans march out, I'll wear it again. You're becoming quite human. (..)1942 Casablanca