Through some tricks of the human eye, we can see colours outside of the normal visual range: Stygian, self-luminous, and hyperbolic colours, and perhaps even combinations like redgreen. These are the impossible colours.
Most of the impossible colours are classed as “chimerical colours.” You can see them by staring at a block of colour until the cone cells in your retina become fatigued. Then you quickly shift to a different block and they overcompensate … and the impossible colour appears.
The Stygian colours are super-saturated but also black. The self-luminous colours seem to glow. The hyperbolic colours look saturated beyond what is normally possible: for example, you may see an orange that is “purer” than the purest orange in the physical world. You can try all of these out on the image above.
More controversial are the combinations of two opposing colours: redgreen and blueyellow, for example. Note that this is not just a blend of the colours – it actually looks like both of them at the same time. Or so some people claim.