It was a brain teaser that took the internet by storm and led people to doubt the colours in front of their eyes. For some, the stripes of #TheDress were blue and black, for others gold and white.
Yet the original photo, posted on the Tumblr social media site in March, was made up of pixels that were either brown or blue.
Now scientists have investigated what has been called the most dramatic example of differences in colour perception ever documented and concluded that it is all to do with the way our brains filter light.
One study of more than 1,400 individuals, including 300 who were newcomers to #TheDress, found that people fell into one of three camps - blue/black, white/gold, and a smaller blue/brown contingent.
What colour stripes participants see may correspond to whether their brains expect to experience artificial "indoor" or natural "outdoor" light, according to US neuroscientist Dr Bevil Conway, from the Massachusetts Institute of Technology.
People who perceive white and gold stripes may be more attuned to daylight while those who see a black and blue dress are more accustomed to indoor lighting, he said.
The brains of "brown and blue" individuals were likely to fall in a bracket somewhere between. "The big open question is what causes these differences in the population," Dr Conway said.
"One framework for understanding why you get these variations is to consider how light is contaminated by outside illumination, such as a blue sky or incandescent light.
"Your visual system has to decide whether it gets rid of shorter, bluer wavelengths of light, or the longer, redder wavelengths, and that decision may change how you see The Dress."
Dr Conway also found that perception differed by age and sex. Older people and women were more likely to report seeing white and gold, while a larger proportion of younger people saw black and blue. His was one of three papers addressing The Dress enigma published in the journal Current Biology.
Another team of psychologists from the University of Bradford in the UK and Giessen in Germany found that the colour confusion probably stemmed from the effects of brightness and the "daylight locus" - the set of natural colours from yellow to blue that we experience during the course of a day.
In the third study, US scientists led by Dr Michael Webster, from the University of Nevada, linked The Dress phenomenon to the eye's evolution in the presence of natural lighting.