Animation Color Snap - parts fade in gray (grey) until last frame

Regarding an animation from within an assembly: How can I make a part fade in (or out) while maintaining its part-level assigned color? The intermediate grey/gray color is unplesant.

I attached a simple example. In reality I'm working with a large assembly and need to fade in/out multiple parts, so it doesn't make sense to mess with each of the individual parts' appearances (again) from within the assembly.

 

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