Because our brains interpret colours and shading relative to their surroundings. That specific blue is on the opposite side of the colour wheel from red, so that relative lack of blue can be interpreted by our brains as red.
Remember that white is all colours present, so white next to white will have more red than white next to blue.
You’d get a similar effect if you stare at a bright blue version of the can for a while and then look at a blank white page or close your eyes. The after image isn’t the same colour as the thing you were staring at, it’s the inverse of that colour.
Because our brains interpret colours and shading relative to their surroundings. That specific blue is on the opposite side of the colour wheel from red, so that relative lack of blue can be interpreted by our brains as red.
Remember that white is all colours present, so white next to white will have more red than white next to blue.
You’d get a similar effect if you stare at a bright blue version of the can for a while and then look at a blank white page or close your eyes. The after image isn’t the same colour as the thing you were staring at, it’s the inverse of that colour.