Wavelength ↔ color
Put in a wavelength and get the color. Pick a color and get the wavelength back out — where there is one to give.
From a wavelength
Nanometres. The eye runs about 380–780; outside that the math still works, the color does not.
From a color
Its dominant wavelength: the pure light it points at, seen from white.
Where it sits
CIE 1931 chromaticity — every color a human eye can seeThe rim of the horseshoe is pure light — one wavelength, nothing mixed in. Everything inside it is a blend, and the further in you go the closer to white.
The straight edge along the bottom is the line of purples. Those colors are real, but no wavelength makes them: they take red and blue cones together with nothing in between. That is why this page converts one way cleanly and the other way only sometimes.
The dashed triangle is sRGB — everything your screen can actually show. Notice how much of the horseshoe falls outside it.