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 she is... is... amazing. She's so smart. She doesn't just see things in black and white. She sees this whole gray area and help me to explore. We just bonded really quickly. At first I thought it was because that's how they're all programmed... no... but I dont think that's the case. (..)2013 Her