Sooooo Emet is a bit interesting.

Most people are going to probably think of him as purple and black because of how he looks as Hades. The final form of Hades has more red than purple though. Either would work, and I would definitely recommend using the colors from Hades, rather than the colors from him being Solus. It's more natural, although if you wanted to associate him with Solus more then you'd probably do something like gold.