Instead of reflecting ambient light, why not measure it and then emit the same amount? It should be indistinguishable (as long as you reconstruct the RGB colors properly), so you can use a regular OLED display.
As far as I can tell OLEDs are 20% efficient or less so you would need an input power of 50 W. Even at 60 deg. N where I live we get over 300 W/m^2 in August so would still need about 15 W of input power to the display.