i tried my experiment with making a black block with a smaller white block inside and then colored text in that. the device simply cant make anything whiter than the gray background.
having seen the microscopic images I thought it might be possible that a lighter background was possible by now i am fairly certain thats as white as it can be. the only way to make white is to leave white eink below the red green and blue sub pixels and they are simply too big to "mix" visually into white especially with a true white subpixel next to them.
i wondered in another thread if a sort of pentile arrangement of the color filter would work better, perhaps even allowing for more white sub pixels. i think ill reach out to a few people to see what they think of that idea.
edit: hmm well it seems this grid of rgbw is already considered "pentile"
but i still think a differetn arrangement of the filter might help brightening the whites