Colour

A construction, not a property.

How it works

Three cone types respond to overlapping ranges of wavelength, and the brain compares their outputs. Colour is the result of that comparison plus assumptions about the light source.

Colour constancy keeps a white shirt looking white indoors and outdoors.
The dress that divided the internet was an ambiguity about the assumed lighting.

Variation

Around one in twelve men has some colour vision deficiency. A minority of women may have a fourth cone type. Perceived colour genuinely differs between people.