If you don't mind power consumption then you can also use reflective LCD display's. Brand like Pixel Qi offer those. They have 3 modes: backlight, reflective,

If you don't mind power consumption then you can also use reflective LCD display's. Brand like Pixel Qi offer those. They have 3 modes: backlight, reflective, and a combination of both. But to be fair the reflective mode always looks monochrome to me.