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'?
(..) BY SIMON & GARFUNKEL PLAYING) My daddy was the family bassman My mama was an engineer And I was born one dark gray morn With music coming in my ears In my ears They call me Baby Driver And once upon a pair of wheels (..)2017 Baby Driver