> Pretty much all colored e-paper screens have much lower contrast than color printing on paper, since they mix colors by using can conventional RGB sub-pixels and darkening them individually, just like regular lit screens, which reduces the amount of reflected light.

Isn't that how color images printed paper works, too? We use inks (often in CMYK coloration, but a galaxy of other options exist) to subtract light from what would otherwise be reflected by a plain white paper.

What makes e-paper screens worse in this way?

They more or less have colored particles hanging around in goop and those get pushed around within a small sealed cell by electrostatic charges, there’s presumably some fundamental limit on the total quantity of the colored particles within the cell that’s quite low. I think modern displays have 4 different colored particles in each cell implying only a small portion of the contents is viewable most of the time. On paper you can have basically 100% saturation of whatever color you want in one area.

> Isn't that how color images printed paper works, too?

No. When you print a piece of paper some color, e.g. red, it will be completely red. But most e-paper screens will only be 33% red (optimistically) and 66% black. This is because physical pixels usually can't change color themselves, only brightness, so you use three of them, and darken the RGB components, to produce a colored pixel.

For displaying white on color e-paper screens you will have three non-dark RGB sub-pixels, but each color component only reflects at most a third of the incoming spectrum, either red, green, or blue wavelengths, while white paper (or monochromatic e-paper screens) will reflect all three wavelengths everywhere.