There has been passionate debate on the Internet over a blue and black dress that to some people - perhaps even the majority - appears white and gold. But what is the reason behind a discrepancy that has caused such division within households and offices?

The story began when Caitlin McNeill posted a picture of a dress that was worn to a wedding, but no one appeared to be able to agree on what color it was. The picture, and debate, went viral.

The physical color of light coming from an object is made up of two components: the color of the object and the color of the light that shines upon it. The combined color can vary widely. For example, daylight is much redder in the evening than at midday. Most of the time, however, we need to know the color of the object, not the color of the illumination - we don't want to think that fruit is ripe in the evening just because the sun is going down. Our visual system has therefore developed to discount the color of the illumination.

Color vision in particular works in three main stages. We have receptors in the eye for just three colors, roughly, blue, green and red. We then make comparisons between the colors in a further three ways: red versus green, blue versus yellow (the sum of red and green for lights) and bright versus dark.

These comparisons are made over different times and different points in space. So if we stare at a blue screen for a long time, when we then view a white object it may appear yellow. Similarly, an orange spot will appear red when surrounded by green, and green when surrounded by red.

Finally, the color signals are passed to the visual cortex, where the brain tries to work out the color of the illumination from all the colors available and then calibrates the colors of the objects according to this illumination color. This is called color constancy, and it's what stops the apparent colors of objects changing during the day.

With the blue dress, it's possible that some people have more blue photo-receptors than other people, or that their color contrast system is biased for or against blue. But the most likely explanation is that color constancy is responsible for the differences of opinion. When the dress occupies most of the image, some people take the blue cloth as their reference for the illumination and so see the dress as white cloth in blue light and the black frills as gold or yellow.

Yellow things don't reflect much blue and so ought to look dark in a blue light. People correcting for a blue light will therefore see the dark frills as yellow. But other people may use the background objects as their white reference and so see the dress as a blue garment in a yellowish-white light. These people will see the frills as dark because dark frills remain dark when compared with a white light.

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Schofield is a lecturer in psychology at University of Birmingham in England.

This article was originally published on The Conversation: http://theconversation.com.