Stygian Blue

There is a blue as dark as black that is also unmistakably blue, and it takes about half a minute to see it.³

No surface can be that colour, no screen can display it and no ink can print it.³

The procedure is simple. Fixate on a bright yellow field for a sustained period, then shift your gaze to a black square.³ The cones carrying the yellow signal fatigue, their sensitivities shift, and the afterimage that follows is in the opposing hue.³ What makes this case different from an ordinary afterimage is the surface it lands on. Black returns almost no light, so the visual system is told simultaneously that there is nothing to see and that what is there is saturated blue. The sensation that results has the darkness of black and the hue of a strong blue at once, a combination that cannot occur physically, because a surface that dark reflects too little light to carry a hue.³

Paul Churchland set out the class these belong to in Philosophical Psychology in 2005, arguing that the Hurvich–Jameson opponent-process model does more than summarise familiar colour relationships: it predicts sensations that lie outside the range any physical object can produce, corresponding to activation patterns the neural network can occupy, and it supplies a method for inducing them.¹ He made the same argument in book form the same year, deriving it from the standard colour-opponency model.² He named three kinds. Stygian colours are dark and saturated at once. Self-luminous colours appear to glow off ordinary paper. Hyperbolic colours look more saturated than any real light or pigment.³

These are not the same thing as the better-known forbidden colours. Those come from holding a boundary between opposing hues stationary on the retina, under which conditions observers have reported seeing reddish greens and bluish yellows.⁴ That is a different manipulation producing a different result, and the two are frequently confused.