This example is rather reminiscent of Adelson's checker shadow illusion, where the typical viewer perceives the tile labelled B to be of a white colour, while that of A to be of a darker grey colour, when in fact the two colours are exactly the same. There are many ways to prove to oneself that the two tiles are indeed of the same colour (using an eyedropper tool, for example), and one that perhaps is most illustrative is by removing the tiles from their surrounding context - and that is the crux of the matter, really, that we perceive in relation to other objects present in our perceptive field. What's also interesting about this color illusion is that besides teaching one about the relative perception of colour, it also exemplifies how the eye disregards more gradual changes in colour - hence the fuzzy, gradient shadows blend in perfectly to create this visual illusion, as the mind adjusts for interpretation of the actual colour of the object.