The technological challenge for researchers working on the next generation of electronic paper is to render color as brightly as traditional paper, without increasing power requirements or end-user costs.
Products and systems mentioned include a technical from Philips called in-plate electrophoretics; here's a paragraph about it from a May 2009 MIT Technology Review
Philips's technique, which is called in-plane electrophoretics, differs in that it involves suspending colored particles in a clear liquid and moving them horizontally instead of vertically. Each pixel is made up of two microcapsule chambers: one containing yellow and cyan particles, the other, below, containing magenta and black particles. Within each microcapsule, one set of colored particles is charged positively while the other is charged negatively.
Another technology worth noting is from the University of Cincinnati Novel Devices Laboratory, which uses a technique called an "electrofluidic display". Their description of it:
Shown at right is our new ‘electrofluidic’ display structure which reduces the visible area of the colored fluid by 2-3X more than that of an electrowetting display. The electrofluidic architecture is further unique from electrowetting displays in driving principles, device structure, potential for bistability, reduced parallax for multi-layer subtractive color pixels, in tight pixel confinement for rollable displays, and in use of water-dispersed pigments instead of oil soluble dyes. We chose the ‘electrofluidic’ nomenclature because the mechanism involves charge induced movement of liquids through microfluidic cavities.