Colour constancy and the dress

How the correction works

The light reaching the eye from a surface is a product of two things: the colour of the surface and the colour of the light falling on it. The eye receives only the product. To recover the surface, the brain uses the whole scene. It compares each patch with its neighbours, looks for the brightest area and for shadows, and draws on experience of what things usually look like. Edwin Land, the American inventor of the Polaroid camera, showed in the 1970s how strongly the surroundings of a patch decide its apparent colour.

The dress

In February 2015 a washed-out phone photograph of a striped dress spread around the world. Some people saw it as white and gold, others as blue and black, and each group found the other hard to believe. The real dress was blue and black. The photograph gave very few clues about the lighting. Viewers who took the dress to be in bluish shadow discounted the blue and saw white and gold. Those who took it to be under warm artificial light saw blue and black. Several research teams published studies of it within months.

Limits of constancy

Constancy is good but not perfect. It works best in natural light, which varies in a predictable way from yellowish to bluish, and worst under lamps with an odd mix of wavelengths. Old sodium street lamps gave out almost pure orange-yellow light, and under them cars and clothes lost nearly all their colour. A camera has no constancy at all unless it is given a white balance setting. This is why a photograph taken indoors can look far more orange than the room seemed to the person standing in it.

Sources and further reading

• Color constancy on Wikipedia
• Lafer-Sousa and others, Striking individual differences in color perception uncovered by 'the dress' photograph, Current Biology (2015)
• Wired: The Science of Why No One Agrees on the Color of This Dress
• The Guardian: #TheDress: have researchers solved the mystery of its colour?