In the case of the dress, the reason some people see it as different colors is not because they're colorblind, which is usually caused by a defect in a person's color cones, nor is it some fundamental difference in color vision, Williams said. "I think the brain has just made a different assumption about how the dress is being illuminated."
For example, if your brain assumes the lighting on the dress is very dim, it will assume the dress itself is highly reflective, or white and gold, Williams said. But if your brain assumes the opposite (that the lighting is very bright), it then makes the judgment that the dress itself must be darker, hence blue and black.
So why don't we encounter these color differences in everyday life?
In real life, the dress would be in a large field of view, with other objects illuminated in the same way. Our brains would be able to separate the garment's lighting from its intrinsic color, Williams said. "It's all about the context," he noted.