Looks to me like a white and gold dress that's being shaded from the ambient light. In reality, it's a black and blue dress with a yellow light cast on it.

Your mind naturally tries to compensate for color shifts caused by light. For example, a white car still looks white to you at sunset and in the moonlight even though it is actually reflecting that red or blue light, not white light (i.e. all colors of light). That's because your eye recognizes the general pallete of the ambient light and makes an interpretation automatically about the colors of the objects you see.

That's happening in this picture too, either correctly interpreting it as black and blue in yellow light or incorrectly as white and gold in shade. But even knowing it's incorrect, changing your brain's interpretation is not easy.