The CIE xy chromaticity diagram is perceptually non-uniform — equal distances do not mean equal color differences

A common mistake when using the xy chromaticity diagram is treating spatial distance as perceptually proportional — assuming that colors equally far apart on the diagram look equally different. They do not: in some regions of the diagram (greens), colors can be far apart while appearing very similar; in others (blues), a small shift causes a large apparent change. The MacAdam ellipses visualize this: equal-appearance regions around any color form ellipses of very different sizes across the diagram. More perceptually uniform alternatives exist (CIE 1976 UCS, CIELAB) but the xy diagram remains common in color space discussion because of its simple derivation from XYZ.

Examples

Two greens separated by the same xy distance as two blues may look far more similar to the human eye. Palettes designed by equally spacing colors on the xy diagram will look unbalanced.

Assessment

Why is spacing colors equally on the xy chromaticity diagram not a reliable strategy for creating a perceptually balanced palette? Name a more perceptually uniform alternative.