Digital image data are coded in a color coordinate system bef where b=0.3·ln B and B, e, and f are Bef coordinates. This coding has high perceptual uniformity with respect to coding errors. A color difference formula is provided, with the color difference defined as 100√{square root over (Δb2+Δe2+Δf2)}. The high perceptual uniformity of this color difference formula is shown using a non-uniformity coefficient developed by the inventor(s). Other features are also provided.