Caitlin McNeill asked her friends to help her identify the colors in this dress. Thus began a nationwide debate over how the brain perceives colors, and the effects of light.

This story was updated at 12:08 p.m. with additional information from experts.

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Quick, what color is this dress?

That's the question 21-year-old singer Caitlin McNeill asked her friends on Tumblr. 

Is it white and gold, blue and black, blue and gold, or some other combination?

In reality, it's a royal blue dress with black lace. But try telling that to the folks who see the dress as white and gold. 

The ensuing debate has spread onto Twitter and into living rooms, dividing families, coworkers and friends into warring camps. 

The sharp divide traces its roots to the way humans perceive color, a trick of light and a function of the way each individual brain is wired. 

Optometry experts say the colors in the photo of this particular dress somehow fall into a neutral area, a one-in-a-million image that falls somewhere in the middle between multiple colors that allows the subtle differences in how our brains perceive color to make themselves known.

According to Wired, the image of this dress hits some sort of perceptual boundary, forcing editors to use Adobe Photoshop in an attempt to draw out the actual colors. But even Photoshop doesn't fully answer the question, because it doesn't account for how the image is white balanced and the effect of background in how we perceive color.

Technically, there are multiple colors in every color, but when there are similar amounts of different colors the human brain attempts to "interpolate a kind of color context for the image, and then spits out an answer for the color of the dress," according to Wired. 

Changing the white balance, or even the background on white certain colors are displayed could completely change the way an average brain perceives color. 

The Associated Press reports that those who subconsciously seek detail in the dress' many horizontal black lines convert them to a golden hue, while the blue disappears into a blown-out white. Experts say the photo's exceptionally warm yellow backlighting triggers this alternative perception.

The dress was worn by the mother of the bride at a Scottish wedding. In an attempt to end the debate, McNeill, who provided music for the wedding in which the dress was worn, says that the dress appeared to be blue and black in person, according to The Independent

Does that settle it? Perhaps not.

The English dress retailer, Roman Originals, has reported a million hits on its sales site since the photo's worldwide distribution Thursday night.