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  1. What is color? Most people would say it's a property of substances. Butter is yellow, the sky is blue (or gray, if you live in Seattle), bears are brown, clouds are white (or gray, if you live in Seattle). This is the "pigment" model of color: If you want green paint, mix blue and yellow. This is actually a pretty robust model. The problem is that it fails to explain color perception in the organism. If we want to understand what it means when we see a color, it's not sufficient just to say, "Well, that's the color of that thing." As we dig deeper to figure out what "color" is, we find another, more powerful, model for color, the one physicists use: Color is a measure of the wavelength of visible light. Thus, "red" light is light of a wavelength within a certain range, as is "blue" light and "violet" light and "yellow" light and "green" light and so forth. In fact, once you learn the "rules" for combining light wavelengths to make other colors (similar but not identical to those for pigments), this model becomes even more powerful than the pigment model. Except there's a huge, gaping hole in this whole theory. If light "color" corresponds one-to-one with light "wavelength", why should combining "red" light with "green" light give you "yellow" light? What you have is red and green, with red and green wavelengths. Those two types of photons do not magically combine into brand new, yellow-colored photons. This observation points the way to a real consideration of what color actually is, which is surprisingly simple: Color is a perception by your brain based on signals from your retina. The reason you see color is because your eyes' retinas have a special type of color-sensitive cell, called a "cone". These cones come in three flavors, each tuned to a certain range of wavelengths: magenta (red), cyan (blue), and green. In reality, each type of cone is sensitive to a large range of wavelengths, so there is lots of overlap between all the cones; but each type of cone is most sensitive to its corresponding "color". So if we see some "pure yellow" light, in the physics sense -- that is, for example, light with a wavelength of 580 nanometers -- the color-sensing cones in our retinas (R for magenta or red cones, G for green cones, B for cyan or blue cones) output something that looks like this: R: ++++++++ G: ++++++++ B: ++++ So here's the key to color! If we can produce the above sensation in an eye, BY ANY MEANS, that eye will see yellow! So what if we show red (magenta) light and green light together? (Remember, the blue-sensing cones will react to some extent, even though they are not "tuned" to those wavelengths.) In such a case, we might well get a profile that looks like this: R: ++++++++ G: ++++++++ B: ++++ Well, well. Whaddya know? Looks like the above profile! (Because I copied-and-pasted it...) So now we know why, on a computer monitor, the "yellow" is actually just red and green. It's all a matter of getting our retinal cells to respond according to a profile. There is no actual "yellow" light there, but that doesn't matter. This is the big challenge currently being fought with LED light bulbs: How do you "tune" the light bulb so that it looks like it's giving off "white" light? True "white" light is a more or less even blend of all visible wavelengths, continuously distributed throughout the spectrum. But LEDs don't show a spectrum; they show only ONE wavelength of light (actually, a very narrow band of wavelengths). So the trick becomes, how can you combine various colors of LED, which output various discrete wavelengths of light, to mimic the effect of white light in the human eye? There are at least two major problems with this: Light that reflects off of various colored surfaces can be changed depending on the wavelength, with some wavelengths being reflected better than others and some absorbed more than others. Thus, even if the light coming off the LED bulb looks "white" to our eyes, objects illuminated by that light might look really weird-colored, not normal at all, because they are not really being illuminated by a continuous spectrum. Different people's retinas have slightly different tunings for the color cones. In fact, the odds are that your two eyes have slightly different tunings from each other, so that you don't see colors quite the same in your left eye as you do in your right eye. So it becomes impossible to select just a few diode wavelengths to produce a white-looking light that looks good to all people.So what has this to do with this list? Nothing, really. I just thought it was interesting. I have a degree in physics and a fairly deep background in biology, yet it took me many years of pondering this to figure this out. (I'm not the brightest LED in the chandelier.) I'm sure this has been understood for a hundred years, but I only realized it maybe ten years ago, maybe less. I do think this has great philosophical value, though. In fact, I think the principle involved is directly applicable to much of what we see in society, in philosophy, in religion, and in our own lives.