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'?
(..) ...and makes your heart ache because of it. I tell you, those voices soared... ...higher and farther than anybody in a gray place dares to dream. It was like a beautiful bird flapped into our drab cage... ...and made those walls dissolve away. And for the briefest of moments... (..)1994 The Shawshank Redemption