In real life, you are right. However, this picture was taken with a camera, which did some auto white balancing internally. Perhaps the camera chose to use the

In real life, you are right. However, this picture was taken with a camera, which did some auto white balancing internally. Perhaps the camera chose to use the yellow background as the reference for white-balancing, causing the less "white" dress a bluish tint. You cannot just consider the natural image here. I am sure that if we got everyone to examine the dress in real life, there would be a lot less confusion. I bet a good part of the confusion is caused by faulty in-camera white balancing.