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.