This is a great reminder that the light that enters our eyes is not the absolute determinant of what we see. Our brain gets involved too: it formulates models based on our knowledge and experience of the world around us, and chooses the one that best fits what our senses are telling us. (Our brains are Bayesian.) But sometimes, two models are equally good interpretations: take for example the classic Necker Cube illusion.
You may see this as a cube with its front face pointing to the left and down, or perhaps one facing to the right and up. If you stare at it long enough, it will probably even flip between the two states. The brain, supplied with ambiguous information, switches back and forth between the two models.
I suspect that's what's happening with this dress: a white and gold dress, or a blue and black dress, are equally valid models for what we see in the photo. But how can such wildly different appearances both be valid? The reason is color constancy: our brains have evolved to see objects in a fixed color, even though lighting conditions routinely change the color of photons reflecting from the object into our eyes. (Without color constancy, the world would appear to change color schemes every time a cloud passed overhead.) It's like the reverse of the Cornsweet illusion: even though the top and bottom segments of this object have the same pixel color (check with an RGB tool, or just hold your finger of the seam), we see them as two entirely different colors: white and black.
In this illusion, the horizon and shadows give us strong clues about the lighting, which causes our brain to interpret the panels as two different colors. In the case of the dress, we have few clues about the actual lighting environment (the photo is a tight closeup), so our brain makes a guess. Some of us subconsciously assume the photo was taken a dark indoor environment with bluish lighting, making the dress appear white and gold. Others assume bright yellow daylight lighting, which makes it appear blue and black. (Anecdotally, it seems the lighting environment in your room where you look at the picture influences the version your brain sees. Try looking at it near a sunlit window.) As always, xkcd explains it perfectly in cartoon form. The center panel is from the actual dress photo; the cartoon dresses in both the left and right panels are the same (RGB) color.
For the record, while I can see the dress as nothing but white and gold, the actual dress is blue and black. (Check out the reviews in that Amazon listing!)
That's all for this week (but you can check out our Because it's Friday series here). See you back here on Monday!