Everything is relative; Our perceptions are driven by comparison…
Just what everyone needs on a Sunday morning, an existential quandary.
It should be noted that there is nothing special about the castle image. This works with any picture. Stare at the negative until your eyes acclimatize to it, then flip it to gray scale and it appears to be in color as long as you don’t move your eyes.
This is somewhat analogous to getting used to a smell or the temperature of a bath or swimming pool; the nerves become somewhat desensitized after a period of over-stimulation. The reason that we see an afterimage is that the rods and cones that weren’t being stimulated are still functioning normally so they send a normal strength signal while the desensitized ones send a weaker signal so the colors that we weren’t looking at get punched up a notch.