I use colour picker across multiple projects on both windows phone and windows 8. Allowing users a choice of colour is a good thing from experience perspective. Whilst I found a number of solutions for Windows Phone, I never really found much for Windows 8.
Last year I ported the Coding4Fun ColorHexagonPicker to use with my TableClock app.Recenly working on Scribble I needed a broader range of colours and I decided to create an appbar control that conveniently lives in AppBar.
I have uploaded the source to Github. Feel free to fork and extent.