Instead of removing the items from the ItemsSource collection of RadComboBox, you should remove them from the List property of the dpClientColor class. This is the recommended way to handle this problem in Silverlight.
Since Silverlight does not support having Binding in Style setters, we have "invented" a workaround that works only for RadControls for Silverlight - ContainerBindings. Using it you could bind the Visibility property of the RadComboBoxItems to a property in your model. Please check the end of the following article:
It demonstrates how to bind the IsEnabled property, but Visibility should be virtually the same, except if you want to bind to a boolean property, you will have to use a converter.
the Telerik team