Since Bondo is an ugly pink colour, we need to paint it something fancy. Whip out some general-purpose primer and spray the area with a couple thin coats. You shouldn't be able to see much of a difference between the Bondo and plastic portions of the mouse body. Let the primer cure for 24 hours, or follow the can's directions.
Next, spray with whatever color of paint you want. I used a glossy Krylon Fusion red spray paint, but anything will work. Use a paint that's made by the same company as your primer to prevent unwanted chemical reactions between different brand formulas.