Here is another impossible colour, one you will clearly see but which cannot be painted with light or pigment. Luminous red, or, as cool people call it, brain red, is clearly, unmistakably red, but it is brighter than white. Which is impossible with light, because white is simply all the wavelengths at full blast, and red is only the red wavelengths with everything else off.
But your brain can see more than your eyes, and brain seeing looks just like regular seeing, in that the images are right there, real as a photograph, real as VHS flicker, real as the sick fleeting yellow of a fluorescent tube powering down.
To see brain red you begin with a blue green and tire out your retinas.
It’s specific, this blue green. It’s bluer than true green, and greener than cyan. It is the shade of sunlit barbicide, as the rays first catch it, in the first instant after you set down the glass. You stare, then the brain red follows.
In the image below you will see a circle of blue green with an x in the middle. Stare hard at the x. You will struggle to keep your focus on the x, and you will notice fringing at the edges of the barbicide. This is your retina tiring, a sign that you are about to see brain red.
Keep your attention as tight as you can. After ten seconds the barbicide will be replaced by white, and you will see a circle of brain red in the middle, brighter than the white. It will look like an idealized Japanese flag. After a while the barbicide circle will return and the animation will loop. Each time you see the brain red it will be brighter still.
Set the glass down. Don’t set the glass down. Whichever.