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 Cheryl Strayed. The ashes of a body aren't really like ashes from a fire. Soft and silky, they're more like... pale pebbles mixed with a gritty gray gravel. I spread most of them around her grave. But some of the larger chunks... I put in my mouth... and I swallowed whole. (..)2014 Wild