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'?
(..) Right. Come here. I can't... I can't... This is why we're here. Unobtanium. Because this little gray rock sells for million a kilo. That's the only reason. It's what pays for the whole party. It's what pays for your science. Now, those savages are threatening our whole operation, (..)2009 Avatar