No I was looking at the wrong areas. To me the tan area between the lines looks darker on the left (next to the black) and lighter on the right, next to the yellow. Ok, but I don't get how this is supposed to explain why people thought a blue and black dress was white and yellow. Doesn't matter tho.

[–] ricecake@sh.itjust.works 6 points 2 days ago

It's essentially highlighting the ambiguity the colors can convey. Because our eyes don't see in isolation from our brains, we don't see based on the actual reflected color, but based on the contrast between those colors and context clues. We essentially have white balance and color correction baked into our vision,which is part of why photos without that look weird. Lacking context you process the colors differently.

In this case people saw a blue and black dress and lacking visual context they either compensated for sunlight or the compensated for shade. The contrasts involved (black/white, blue/yellow) are because opposite compensations maintain contrast while changing brightness.

This image has someone wearing the dress photographed with the white balance specifically off so that you can maybe see what other people were implicitly correcting for.