The MouseOver animations are contained in the VisualStates just as you pointed out. So, in order to change Foreground on mouse over, you will need to modify the Style (to be more specific, the brush that the mouse over state uses) and not the ContentTemplate.
Since you cannot set the Foreground property in a ContentPresenter, what you can do is to generate the ItemContainerStyle for the RadComboBox and then change the ContentPresenter to a ContentControl. Then, you are free to specify a custom Foreground brush for that ContentControl in the MouseOver state.
I hope this helps.
the Telerik team
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