You can achieve this by editing the template of the RadCombobox and then find NonEditable ComboBox and then set the Foreground of
Then apply the style to all RadComboBoxes and set the binding for each of them in its XAML. I have attached the template of RadComboBox for Office_Black theme. You can get the templates of other themes from your installation. The path is this:
C:\Program Files (x86)\Telerik\RadControls for Silverlight Q2 2011 SP1\Themes
Hope this works for you. Let me know if you need further help.
All the best,
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>