If you want to change the highlight and focus colors you should edit the control style. Right now it not as easy as it should be but it is doable.
You can read more about styling RadControls for Silverlight from here:
For the next release all controls will be fully editable in Expression Blend 2 SP1 ( from Object->Edit style->Edit Copy).
Let me know if you need more help.
the Telerik team
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