The CIE chromaticity diagram represents a CIE XYZ color space projection to xyY and its approximated RGB color gamut defined by a working color space. The gamut boundary is based on CIE Standard Colorimetric Observer Data. Two sets of colorimetric data, CIE 1931 2° and 1964 10° observer data, are provided, from 360 to 830 nm, sampled at 5 nm.
Each working color space defines the coordinate values of primary and white point illuminants. These values are used to construct the conversion matrix from xyz to RGB and apply it in order to approximate RGB colors.
The whole gamut represents all the colors that human eyes can perceive. The working color space defines the system gamut, which is usually much smaller.
Different working color spaces use different types of white point chromaticities. The NTSC system uses "C". CIE color space uses "E", the CIE equal-energy illuminant. Other color spaces use "D65" (color temperature 6500K).
The "system gamut contrast" slider makes the colors outside of the gamut darker, so that the gamut will be more visible.