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The way I imagine it is having a framed newspaper clipping, and laying a piece of colored film on top of the glass. Without the film, white light hits the paper and bounces back with a certain efficiency. Add a colored filter, and only the red (or whatever) fraction of light passes through to be reflected. You're losing most of the light, so adding filters reduces brightness, just like sunglasses do. But the screen under the filter is the same as always, so it's bistable and will show the last screen even without power, and power requirements are no higher, and it's not LCD. A colored screen will look darker than a b&w one, though, so you might want more light to read by.
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