There are two ways to do this:
1) Use regular RadDiagramShape with custom style and an attached property for the circle's color (if you want to be able to use different colors)
2) use custom RadDiagramShape with custom style and an additional dependency property for the color.
I've attached a sample project so could you please examine it and see if this is what you're looking for.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.