The mother of the bride wore white and gold. Or was it blue and black? The dress in a photo from Caitlin McNeill’s Tumblr site. From a photograph of the dress the bride posted online, there was broad disagreement. A few days after the wedding last weekend on the Scottish island of Colonsay, a member of the wedding band was so frustrated by the lack of consensus that she posted a picture of the dress on Tumblr, and asked her followers for feedback. “I was just looking for an answer because it was messing with my head,” said Caitlin McNeill, a 21-year-old singer and guitarist. Within a half-hour, her post attracted some 500 likes and shares. The photo soon migrated to Buzzfeed and Facebook and Twitter, setting off a social media conflagration that few were able to resist. As the debate caught fire across the Internet — even scientists could not agree on what was causing the discrepancy — media companies rushed to get articles online. Less than a half-hour after Ms. McNeil’s original Tumb