I had a phone (Nokia N73) which had a clock app that ran while the LCD backlight was turned off. In most lighting conditions the contract between the black background and the dim gold colour clock face was enough to read it. Something like that would be ideal for a super low power device such as a solar clock or radio.
How about some kind of holographic display? You can get very expensive full colour ones where you see a different image depending on the angle you look at it from. They use them in some high end cars so that a single monitor can display sat nav data for the driver and a DVD movie for the passenger.
The only other suggestion I have is that if it must be backlit then maybe instead of using just one RGB colour you could put a row of say 5 RGB LEDs along the top and the bottom. That way it would be possible to vary the colour across the display.