You might be able to do it with something less than 1/4 the b/w resolution, by using, say, half the pixels for black, then distributing the other three colors over the remaining half. You'd have lower resolution for color images, but text would crisper and cleaner. So if you could double the pixel resolution, you could add color without decreasing b/w resolution.
OTOH, I'm not sure this would actually work at all. When you print a halftone, you can actually overlap colors. You can physically print Cyan over the Black. But with e-ink, each pixel has to be a particular color. So you couldn't have a black screen if only half of your pixels have the option of displaying black. It would be gray at best.