I have been following with interest discussion of the blue dress among the psych folks. Laura Truroiu told me early on that people's disparate reactions means it is ambiguous like the necker cube:
The necker cube. Which of the two middle corners are closer?
Unlike the necker cube, most people can't switch back in forth easily but instead have a strong perception of the dress. Laura said this implies people have different "priors" be they innate for that individual or environmental. (A "prior" is our contextual or built-in assumption used to resolve ambiguity like "light comes from above", or "humans are 1-2 meters tall"). I believe she is correct, and there is a lovely long piece on this for those that want to dig in on that.