Atheists how would you prove whether this is an illusion or not?

Not so with shapes because you could ask rational questions like "how many sides does it have?" "what are the angles between each side?" The equations for finding the area of a circle are different to that of a triangle. Things like colours are more subjective though and it can be shown through tests that one person sees a different colour to others (I'm colour blind, so I know this is the case. My wife and I see different colours.)