The photo in debate (C) transforms into gold and white (L) or black and blue (R) in different white balance option. [Photo from web]

Taylor Swift saw the dress was "obviously" blue and black. "What's the matter with u guys, it's white and gold," countered Julianne Moore. Kim Kardashian, never one to miss a trending topic, reported she was seeing gold but to husband Kanye West, it was solidly black and blue. "Who is color blind?" Kardashian asked the twitterati.

The answer, says Hardiman-McCartney, is that every viewer seeing either set of colors is right.

He says the exceptional bar-code style of the dress, combined with the strongly yellow-toned backlighting in the one photo, provides the brain a rare chance to "choose" which of the dress' two primary colors should be seen in detail.

Those who subconsciously seek detail in the many horizontal black lines convert them to a golden hue, so the blue disappears into a blown-out white, he said.

Others whose brains focus on the blue part of the dress see the photo as the black-and-blue reality.

"There's no correct way to perceive this photograph. It sits right on the cusp, or balance, of how we perceive the color of a subject versus the surrounding area," he said. "And this color consistency illusion that we're experiencing doesn't mean there's anything wrong with your eyes. It just shows how your brain chooses to see the image, to process this luminescence confusion."

The photo produced a deluge of media calls Friday to the Tumblr reporter, 21-year-old McNeill, who calls the seemingly endless phone calls "more than I've received in the entirety of the rest of my life combined." She says the photographer, who is also the mother of the bride, never wanted the publicity.

There's one clear winner: English dress retailer Roman Originals, which has reported a million hits on its sales site in the first 18 hours following the photo's worldwide distribution.

"I can officially say that this dress is royal blue with black lace trimming," said Michele Bastock, design director at Roman Originals.

She said staff members had no idea that the dress, when shot in that singularly peculiar light, might be perceived in a totally different color scheme. Not until Friday anyway, when they arrived at work to field hundreds of emails, calls and social media posts. They, too, split almost 50-50 on the photo's true colors.

All agreed, however, the dress for the Birmingham, England-based retailer was likely to become their greatest-ever seller. The chain's website Friday headlined its product as "(hash)The Dress now back in stock _ debate now."

"Straightaway we went to the computers and had a look. And some members of the team saw ivory and gold. I see a royal blue all the time," she said. "It's an enigma ... but we are grateful."