There's a pretty solid theory that the perception of the dress is related to your circadian rhythm and how your brain interprets light - you're either a lark, or an owl (early or late). If you're a lark, your brain expects things to be lit by sunlight. An owl expects incandescent light. So in the absence of other clues about the correct color of the dress, your brain decides the color based on its expectations for how colors look in the kind of light you're used to getting.