This is exactly the thing.

Whatever the dress may be in reality, the photo of it that was circulated was either exposed or twiddled with such that the pixels it's made of are indeed slightly bluish grey trending towards white (i.e. above 50% grey) and tanish browny gold.

That is absolutely not up for debate. Those are the color values of those pixels, end of discussion.

Edit to add: This entire debacle is a fascinating case of people either failing to or refusing to separate the concept of a physical object versus its very inaccurate representation. The photograph of the object is not the object: ce n'est pas une robe.

The people going around in this thread and elsewhere putting people down and calling them "stupid" or whatever else only because they know that the physical dress itself is black and blue based on external information are studiously ignoring the fact that this is not what the photograph of it shows. That's because the photograph is extremely cooked and is not an accurate depiction. The debate only exists at all if one party or the other does not have the complete set of information, and at this point in history now that this stupid meme has been driven into the ground quite thoroughly I should hope that all of us do.

It's true that our brains can and will interpret false color data based on either context or surrounding contrast, and it's possible that somebody deliberately messed with the original image to amplify this effect in the first place. But the fact remains that arguing about what the dress is versus how it's been inaccurately depicted is stupid, and anyone still trying that at this late stage is probably doing so in bad faith.