stianhat wrote:The sky is blue because of the colors it *lacks* not the colors it has - we percieve chlorophyll filled leaves as green because they lack the red light and correspondingly, as someone was close to earlier, we see the sky as blue because it lacks red light, but also has violet light ( violet = not yellow ) (not red = green). the resultant mix of (not yellow) + green = blue. The blue light that is already there just adds to this.
It occurred to me why this troubles me: given that the cones in the human eye are red/green/blue, shouldn't the opposite of violet and yellow be green and blue, respectively?
Unfortunately the physiology of color perception isn't consistent between the cones in the eyes and the nerves which carry those signals to the brain. The signals our brain receives aren't "how much red", "how much green", and "how much blue", but "how much more red than green, or vice-versa", and "how much more blue than red or green, or vice-versa". Quoth the wiki again:
Besides the cones, which detect light entering the eye, the biological basis of the opponent theory involves two other types of cells: bipolar cells, and ganglion cells. Information from the cones is passed to the bipolar cells in the retina, which may be the cells in the opponent process that transform the information from cones. The information is then passed to ganglion cells, of which there are two major classes: magnocellular, or large-cell layers, and parvocellular, or small-cell layers. Parvocellular cells, or P cells, handle the majority of information about color, and fall into two groups: one that processes information about differences between firing of L and M cones, and one that processes differences between S cones and a combined signal from both L and M cones. The first subtype of cells are responsible for processing red–green differences, and the second process blue–yellow differences. P cells also transmit information about intensity of light (how much of it there is) due to their receptive fields.
(S cones are the ones that detect short-wavelength blue light, M cones detect medium-wavelength green light, and L cones detect long-wavelength red light)
So instead of a color wheel with red, green, and blue each 120 degrees apart from each other, our mental model of color is more of a color square with red at the top, green at the bottom, and blue on the right, with the yellows (between red and green) stretched out across the left side, and everything from cyan to magenta squished together on the "blue" side. (Flip and rotate this square as you like, directions arbitrarily chosen). In essence, our brains think in four primary colors: red, green, blue, and not-blue aka yellow.
All that aside, violet is not equivalent to magenta or any of the purples (colors on a color wheel between red and blue): violet is basically "ultrablue", a blue so blue that the M cones aren't getting any of the stimulation they get at the frequency where S cone response peaks (which is why violet appears dark, it's hardly stimulating any of the cones, but the S cones gets most of what little there is). We think of violet as like a purple because, sadly, the L cones pick up their sensitivity again down at those short wavelengths, so this "ultrablue" seems, perversely, a little bit reddish to us.
On a highly tangential note: it bothers me on an aesthetic level that our color vision is such a messy hodge-podge. In my fictional universe (see sig), I have an alien species which sees colors on a continuous spectrum of frequencies, and colors which go well together are ones with harmonic frequencies, much like our human hearing perception finds that sounds with harmonic frequencies sound good together. If only human vision were so elegant...