I suspect this is an effect of our Foveal Vision: http://en.wikipedia.org/wiki/Fovea_centralis http://en.wikipedia.org/wiki/Fovea_centralis We only see things

I suspect this is an effect of our Foveal Vision: http://en.wikipedia.org/wiki/Fovea_centralis http://en.wikipedia.org/wiki/Fovea_centralis We only see things in the centre of the eye crisply and the effective 'resolution' reduces the further away we get from this point. Thus, perhaps, the extra colour is perceived as more screen rather than coloured wall.