The CIE xy chromaticity diagram projects 3D XYZ color into 2D by separating hue from brightness

Dividing XYZ by (X+Y+Z) yields normalized coordinates x, y, z summing to 1, allowing z to be dropped. The resulting 2D xy plot shows all human-perceptible chromaticities as a horseshoe: spectral colors on the curved boundary, white near the center. The straight ‘line of purples’ closes the horseshoe — these colors have no spectral wavelength. The diagram is widely used to compare color space gamuts (triangles with primaries as vertices). Caution: the xy diagram is perceptually non-uniform — equal distances do not represent equal perceptual differences.

Examples

sRGB gamut triangle on xy: vertices at (0.64,0.33), (0.30,0.60), (0.15,0.06). Display P3 has a larger triangle.

Assessment

What does it mean for a color to lie outside a color space’s gamut triangle on the xy diagram? How does a renderer handle such colors?