To change the color of the ComboBox you have to edit its style. You can do that by editing the control in Blend Expression. Right-click on the ComboBox and from the menu choose Edit Template-> Edit a Copy. Blend will generate for you all styles and templates for the ComboBox.
Attached you will find a sample project in which that is illustrated. Please mind for the "NOTE:" comments which point to the place where you have to make the changes.
If you have further questions please feel free to contact us again.
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