In the Theory of Everything board in the MKaku Forum, 8LPF16 asked about colours (see "question" thread), and the discussion got onto the relationship between colour perception, spectral intensities, and the colour systems used in printing, TVs and computer monitors, etc. 8LPF16 asked some questions I couldn't answer, and am myself curious about. Can anyone here help? 1) Violet, or purple (English colour names are far from unambiguous!) is a small range of wavelengths around ~400 nm. However, the 'blue' cones have only limited sensitivity to these wavelengths, and the 'green' and 'red' ones virtually none. On my computer screen I get a purple colour with ~60% blue and ~40% red (using the RGB system). The intensity of the colour doesn't seem to matter much, until it become so faint that all colours get washed out (i.e. only the rods are stimulated) What's happening, in terms of rod and cone activity, when you look at the violet part of the rainbow? 2) Magenta (as in the M in the CYMK system) is clearly not 'a small range of wavelengths around x nm', which is how any colour in the rainbow could be described. In this case, intensity does seem to matter; as it gets faint (or dark), magenta appears to me to become more purplish. What is the spectrum of a magenta-coloured object? (This may be a question better directed to someone in a technology board) LPF also asks some other interesting questions; perhaps she'll post them here.