There was one big issue though. It only worked with the default accent colors in Windows Phone 8.
I've just pushed a new version to GitHub that is both forwards and backwards compatible.
It now works on both Windows Phone 7.X and Windows Phone 8.
It also works with operator and manufacturer defined accent colors. In fact it will work with any accent color whatsoever (yes, even for the people who've hacked in their own colors) as it now determines the compliment on the fly, rather than relying on a precomputed list of compliments.