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'?
(..) But you can't build all of them at once. If anybody has black parts, I need them, okay? And sometimes, very, very dark gray. - Use the yellow bricks! - Hey, guys, can I help? - No! It has to be this way! - No, I need that! Anyone know what this (..)2014 The Lego Movie