By Andrew Schofield, University of BirminghamThere has been passionate debate on the internet over a blue and black dress that to some people â€“ perhaps even the majority â€“ 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 colour it was. The picture, and debate, went viral. The physical colour of light coming from an object is made up of two components: the colour of the object and the colour of the light that shines upon it. The combined colour 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 colour of the object, not the colour of the illumination â€“ we donâ€™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 colour of the illumination.Colour vision in particular works in three main stages. We have receptors in the eye for just three colours, roughly, blue, green and red. We then make comparisons between the colours 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 colour signals are passed to the visual cortex, where the brain tries to work out the colour of the illumination from all the colours available and then calibrates the colours of the objects according to this illumination colour. This is called colour constancy, and itâ€™s what stops the apparent colours of objects changing during the day.The two-tone dress that sparked global debate.Joe Giddens/PA WireWith the blue dress, itâ€™s possible that some people have more blue photo-receptors than other people, or that their colour contrast system is biased for or against blue. But the most likely explanation is that colour 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 donâ€™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.This article was originally published on The Conversation. Read the original article.