Chromaticity

Chromaticity is the colour component of a light spectrum independent of its intensity. Two lights with the same chromaticity differ only in brightness — they’re “the same colour” in the conventional sense.

The standard representation is the CIE 1931 xy chromaticity diagram: a horseshoe-shaped 2D plot where every visible colour at any intensity maps to a single point. White (D65 daylight) sits near (0.31, 0.33). Pure red, green, and blue primaries sit at three corners; mixing produces every colour inside the triangle formed by those primaries.

A colour space is defined largely by its primary chromaticity points. sRGB picks its three primaries at one set of (x, y) coordinates; Display P3 picks them further out, especially the red, expanding the achievable gamut. The chromaticity diagram shows visually which colours each space can represent — the larger the triangle, the wider the gamut.

Practical use: when picking a colour space for an asset (web design, print, video), the chromaticity diagram is the chart that shows whether the space covers the colours you need. For print, CMYK’s achievable gamut is a sub-region of sRGB’s, which is why some sRGB colours simply can’t be reproduced on a printer regardless of technique.