I came across very nice illustration of H.P.Lovecraft's "The Color out of Space" here:
http://elfwood.lysator.liu.se/loth/i/c/ickessler/coloroutofspace.jpg.html
Well, how can one reproduce the color which "does not belong to the normal spectrum"? (of course I know there is no such thing)
One possible way may be to use diffraction grating. Imagine the surface of a CD: the color depends on the angle you look it, and two eyes perceive different colors, which is confusing and cannot described by the normal spectrum. I think that the garment of "Saruman of Many Colours" exploited this effect.
Another way may be to use rapidly changing hues, so fast that one can't recognize each hue, as shown above.
The visual perception and recognition process in the human brain are very subtle, and one can deceive them so easily. Probably it may be even possible to reproduce images of indescribable, ineffable cosmic horror.
