[Kevan]

By definition windows in a model car are supposed to be scale versions of 1:1 scale cars and in the UK these must be:

Windshields - must be over 75% VLT (Visible Light Transmission) - (not blocking more than 25% of light).
Front side windows - at least 70% VLT (not blocking more than 30% of light).
Back side windows - no restriction on VLT.
Rear window - same as back side windows, there is no restriction on VLT.

So strictly speaking you can have black rear and back side windows but black anywhere else isn't actually a window.