Somehow, a white-painted window just doesn't pass very convincingly for a lit window. It doesn't 'throw' light at you the way a lamped window does. Try this:
Paint the window(s) in two steps. First paint a shiny layer, such as an aluminum or stainless steel metallizer paint. This will act to throw some ambient light back at you. After buffing, overpaint it with a thin, tinted transparent layer. The tint can be yellowish or bluish, depending on the color of the interior light you're trying to simulate. A teencey-tiny bit of white paint mixed into the transparent paint can help soften the effect.