Well, to answer the OP's question, I would say the boldest color you can get on a Ghost is Dark Indigo. It's deep metallic Purple. I know that this doesn't sound all that bold, but the Ghost is not part of Rolls-Royce' Bespoke Program and as a result, cannot be special ordered in crazy, personalized colors.
That is gorgeous! Quite a show stopping and ballsy color option. Thank you for sharing the photo.