It could be simultaneously both the biggest and the smallest controversy to hit the internet since, well, ever. What colour is the dress? A Tumblr user uploaded a picture of a dress, saying that they were having an argument about what colour it is. Is it white and gold or blue and black? Buzzfeed republished it and it broke even their records for traffic.
In fact the dress is blue and black.
So why does the picture appears white and gold to some people, and blue and black to others?
There are already many explanations, and most are wrong. But fear not, New Scientist can explain it.
You've probably seen optical illusions like this one above.
Believe it or not, the two squares, A and B, are exactly the same shade of grey. Because there are cues in the image telling you to imagine that B is in shadow, you interpret it as a light coloured square, made darker by being in shadow.
But as Erin Goddard from Macquarie University in Sydney, Australia, points out, a similar trick can be done with colours too. Look at this version below:
Both squares are grey, but you see one as yellow and one as blue. It happens because in both cases you unconsciously correct for what colour you think the source of light is.
We do that all the time to get by in the world: the reason you see a piece of white paper as white regardless of whether you're outside (under the blue sky) or inside (under red-tinted candle light) is because you shift the colour of the paper in your mind back to white – you white balance it. Or in technical terms, you "discount" for the "colour of the illuminant".
In the illusion above, on the left you correct for a light source you think is yellow, making the grey square appear blue. And on the right you do the opposite.
So now to that dress. The key thing is that we are correcting for an imagined light source, just as in the example above. But there are two features of this picture that make it very difficult to interpret, which means people are likely to see it differently.
Firstly, the dress is actually a complicated mixture. If you find the RGB values of the gold/black, they come out as a "yellowish/gold/brown" says Bart Anderson from the University of Sydney. Meanwhile, the "white/blue" is a mixture too. "If you look at the image itself – the colours in the image – they are gold and very light blue," he says, clearly taking a side in the controversial dispute.
The second feature that makes it ambiguous is that there are not many cues in the picture to tell you how to interpret the colour of the light source. You can't even tell if it's in shadow or not, which could make all the difference, says Goddard.
"In addition to making things darker, shadows often 'change' the colour of part of a scene – eg if there's a sunny scene, the direct sunlight is quite yellow, but areas in shadow are mostly lit by skylight, which is quite blue," she says. "Artists know about it – they know to add blue to shadows to make them more convincing."
Without cues telling you how to correct for the light source, people can be left fumbling and possibly just randomly fall into one category. And once you see it that way, it's hard to change.
"It's actually consistent with a few different interpretations but rather than seeing it as 'oh, it could be this or it could be that' you tend to just make a guess and stick with that until you've got evidence to the contrary," says Goddard.
It can then be very surprising to hear that it's not the same for everyone. Goddard says a similar thing happens with the well-known spinning ballerina. You can see it either way, but it once you see it one way or the other, it's very hard to switch.
Anderson says that feature of this illusion struck him. He started off as a white/golder. In fact, when New Scientist first contacted him he thought it was a hoax. "Do either of you see anything remotely black about this dress?" he asked me and another researcher over email. But after playing with the image for a while switched to a black/bluer. "I can't get my original percept back. Usually when this happens, I can see both. So it's very interesting."
But it could also be something to do with people's internal biases, says Anderson. "People may have different estimates of the colour balance – ie, what counts as 'neutral'," he says. Since this illusion so strongly splits people into different camps, it could be interesting to learn about the nature of those inherent biases. "Although it's not clear how it will tell you much more than that yet," he says.
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