Gamut rings are two‐dimensional representations of three‐dimensional CIELAB color gamut volumes. Recently developed for visualizing gamut volumes of self‐luminant (emissive) displays in darkroom conditions, they can also be applied to reflective displays with integrated frontlighting. Frontlights provide illumination in low light conditions and also compensate for optical losses when reflecting ambient light. Since the frontlight is essentially fixed compared to the ambient illumination, this mode of operation is effectively “emissive.” Perceived CIELAB colors of reflective displays with frontlights can thus be treated as superposition of the spectral radiance from the “emissive” contribution and the reflected spectral radiance from ambient illumination. The CIELAB calculation will depend on whether the “emissive” or reflective mode is dominant for a given ambient illumination. CIELAB gamut ring diagrams of electrophoretic color ePaper were determined from laboratory measurements of the displays' “emissive” and reflective spectral characteristics, combined with specifications of realistic ambient indoor and outdoor viewing environments. Gamut ring diagrams demonstrate the benefit of color pigments instead of color filter arrays in both self‐luminant and reflective display modes. Interpolation of envelope colors from display primaries, commonly used in emissive displays, underestimate gamut volumes of color ePaper because reflective display primaries are not additive but subtractive.

Gamut Rings visualize CIELAB gamut volumes of color ePaper in ambient light. Integrated frontlights provide illumination in darkness and compensate ambient light reflection loss. CIELAB requires a different reference white for self‐luminant and reflective display modes. Color interpolation based on additivity assumption will give erroneous gamut volumes of color ePaper.