This means it shows the range of colours achievable by combining red, green, and blue light in varying proportions, not all possible colours imaginable. Some chromaticity diagrams may include colours that are technically visible under specific conditions (e.g., high intensity) but are not typically seen by humans under normal viewing conditions.
The two axes in a chromaticity diagram, typically labelled x and y, represent the proportions of red, green, and blue light needed to produce a specific colour within the model’s gamut.
The most common diagrams, like the CIE 1931 xy diagram, display the entire range of hues (at varying levels of saturation) that a human observer can perceive under ideal conditions.
The scale on each axis of chromaticity diagrams used for technical purposes aligns with the range of colour values (chromaticity coordinates) described by the CIE (1931) XYZ colour space. This enables them to accurately depict colour spaces in a manner consistent with a comprehensive and internationally recognized chromaticity coordinate system.
Some chromaticity diagrams show the smaller range of other colour spaces so that the range of colours that can be reproduced by equipment such as cameras, digital screens and printers can be compared.