The spectral locus is highlighted in this one (for clarity), https://en.wikipedia.org/wiki/CIE_1931_color_space#Chromaticity_diagram https://en.wikipedia.org/w

The spectral locus is highlighted in this one (for clarity), https://en.wikipedia.org/wiki/CIE_1931_color_space#Chromaticity_diagram https://en.wikipedia.org/wiki/CIE_1931_color_space#Chromatic... That outer *bolded* edge is monochromatic light, as perceived by humans. The number labels are the wavelength in nanometers.