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.