The screen really does not let a lot of light shine through. There is a reflective coating (imagine aluminum foil) that reflects back. We did try to use the opacity of screen with high power leds for some usecases but it did not work. I think you need to look at E Paper optical characteristics as you would look at 1 mm thick cardboard.

The best way to fix optical characteristics is to play with the substrate and layers in front. Like adding a very good frontlight really helps with contrast even in non-dark conditions.