Starting on February 26, 2015, millions of people viewed an accidental optical illusion that went viral on the Internet. The popularity of The Dress illusion was due in part to the fact that the public was split between people who can see the illusion and people who can’t. It all depends on whether you see the dress as white and gold or blue and black.
The actual color of the pixels in the lighter part of the dress are blue, and that is all that some people can see, so for these viewers there is no illusion. Others see the lighter part of the dress as white, and for them what seems to be happening is an illusion similar to the banana illusion. That is, their brains perceive the dress to be in a blueish shadow, and “correct” the blue color to white. Some people report being able to switch from seeing white and gold to seeing blue and black.
In the proof below, both circles contain the exact same pixels. For people who see white and gold, this should illustrate that the light colored part of the dress is actually blue.
For those who never saw white and gold in the first place, the images below are the best illustration I have found of how blue and black colors can appear white and gold.