As a reflective display, you want to optimise for the percentage of useful light reflected back to the user. Switching the white (or black) out for another colo

As a reflective display, you want to optimise for the percentage of useful light reflected back to the user. Switching the white (or black) out for another colour will necessarily reduce the contrast ratio of the display, which is of course not ideal when you don't have much light to work with in the first place (as it's a reflective display after all). That's why reflective LCD displays were pretty much a disaster, since the LCDs require a polarisng filter which absorbs a large portion of the light, and reflective colour LCDs more so, because that multiplies with the effect of the colour filters. Of course, you could simply make a matrix of tri-tone capsules, with white, black and colour. Then you have the contrast ratio and the colour space advantages. I don't know why this hasn't been done, I'm not an expert on e-Ink!