Although that dress is really blue and black, it has become the subject of a raging internet debate since yesterday thanks to poor exposure and white balance. It has also sparked some serious discussion about how color vision works.
Why is that?
Color vision is weird, and it seems there's no consensus regarding why this particular debate is happening yet. One thing not debated is that our perception is affected by the environment an image or object is presented in. The background behind an object (or the color of a webpage an image is presented on, there's a reason this site is dark) and the light source an object is seen under affect its perception (metamerism makes that even weirder).
Let's get back to that "why?" We know perception can be skewed by the factors described above but why is it that people are so divided in how they perceive this image? It seems as though something is causing different people to "white balance" the image differently.
A lot of people are jumping in to the discussion on why this is happening and just like the colors people see, their explanations don't all agree with each other. Does it have to do with the retina itself or is it something in how the brain is processing the signal? Some seem to be suggesting it has to do with the spectral sensitivities of the rods versus cones or whether people have more cones than rods (or vice versa).
Personally, I'm not buying that because the average human eye has quite a few more rods than cones (~120 million rods vs ~6-7 million cones, however they are not all distributed uniformly about the retina) and everything I ever learned about color vision indicated that the color signal from the eye is derived from comparing the signals between the different types of cones. That signal is then processed by the brain, producing the final color perception phenomenon. It seems more likely to me that this has something to do with the brain not having adequate context when analyzing the signal from the eye and in different people it's jumping to different conclusions.
The color debate is causing color vision researchers to ask some deeper questions about the underlying processes. This Vice article interviews a color vision researcher about the dress and is a fun read. I'm curious to see what the experts figure out as a result of this.