What colour is the dress? It's both.
Perhaps by now you are sick of the dress that 3/4 people see as "white and gold" and 1/4 people see as "dark blue and black." If you haven't seen it, it's easy to find. What's amazing is to see how violent the arguments can get between people because the two ways we see it are so hugely different. "How can you see that as white????" people shout. They really shout.
There are a few explanations out there, but let me add my own:
- The real dress, the one you can buy, is indeed blue and black. That's well documented.
- The real photo of the dress everybody is looking at, is light blue and medium gold, because of unusual lighting and colour balance.
That's the key point. The dress and photo are different. Anybody who saw the dress in almost any lighting would say it was blue and black. But people say very different things about the photo.
To explain, here are sampled colour swatches from the photo, on white and dark backgrounds.
You can see that the colours in the photo are indeed a light blue and a medium to dark gold. Whatever the dress really is, that's what the photo colours are.
We see things in strange light all the time. Indoors, under incandescent light bulbs, or at sunset, everything is really, really yellow-red. Take a photo at these times with your camera set to "sunshine" light and you will see what the real colours look like. But your brain sees the colours very similarly to how they are in the day. Our brains are trained to correct for the lighting and try to see the "true" (under sunlight) colours. Sunlight isn't really white but it's our reference for white.
Some people see the photo and this part of their brain kicks in, and does the correction, letting them see what the dress looks like in more neutral light. We all do this most of the time, but this photo shows a time when only some of us can do it.
For the white/gold folks, their brains are not doing the real correction. We (I am one of them) see something closer to the actual colour of the photo. Though not quite -- we see the light blue as whiter and the gold as a little lighter too. We're making a different correction, and it seems going a bit the other direction. Our correction is false, the blue/black folks are doing a better job at the correction. It's a bit unusual that the the results are so far apart. The blue/blacks see something close to the real dress, and the white/golds see something closer to the actual photo. Hard to say if "their kind" are better or worse than my kind because of it.
For the white/gold folks, our brains must be imagining the light is a bit blueish. We do like to find the white in a scene to help us figure out what colour the light is. In this case we're getting tricked. There are many other situations where we get our colour correction wrong, and I will bet you can find other situations where the white/golds see the sunlit colour, and the black/blues see something closer to the photograph.