If we have understood you correctly, you want to change the background color of the mask when the IsReadOnly property is set to true. If this is the case, you can extract and edit the default template of the control. In the extracted template, you can navigate to a VisualState with x:Name="ReadOnly"
and set your custom color. If we are in the wrong direction you can elaborate more on what exactly you are trying to achieve in your application. You can send us pictures with the desired result. This way we can better understand your approach.
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