Harmonic Coloring: A method for assigning colors to musical pitches

The color wheel doesn’t really wrap around. We just take a cut of the visible spectrum, and overlay the red and the blue ends. We don’t perceive periodicity in light; near infrared is not one octave lower than violet light. The color range we see is just one that happens to be most useful in a nitrogen/oxygen atmosphere under a Class G sun.

We are often told that the color of light is determined by its wavelength. So why do violet/purple appear as if they were smooth interpolations between red (high wavelength) and blue (low wavelength)?

It's a weird artifact of how the brain reconstructs wavelength from the measurements taken by the retina.

The retina has three types of color-sensitive cells (cones). Each one carries a light-absorbing protein that absorbs only certain wavelengths of light. Photopsin I has maximal absorption around 530nm (a sort of yellowish green) and some absorbption towards higher wavelengths, up to 700nm (the red end of the spectrum). Photopsin II has maximal absorption around 530nm as well (so it's green too), but unlike Photopsin I, its absorption drops off rapidly, and does not go toward red. Photopsin III, however, peaks in low wavelengths, around blue.

Even though the peak responses of Photopsin I and II are very similar, the brain can use absorption ratios to detect wavelength.

If your Photopsin I is absorbing some light, but your Photopsin II isn't, the brain deduces that your retina must be being illuminated by red light.

If both Photopsins I and II are absorbing a lot of light, the brain deduces that you're seeing some shade of green (this is why we can tell greens apart better than other colors: we have two receptors for it, but only one for the other colors).

If only Photopsin III is absorbing light, but the other two aren't, your brain deduces that your retina is being illuminated by high frequency blue light.

Other color perceptions arise as combinations of different wavelengths. If all your Photopsins are absorbing light, the brain deduces that you're seeing a combination of all visible frequencies, white light (this is why prisms can split white sunlight, and why we have rainbows). Similarly, If only Photopsins I and III are absorbing, you must be seeing a combination of red and blue light, which is what you see as purple. And that's why purple appears to interpolate smoothly between red and blue.

Okay, but where does violet fit in? Violet isn't purple (a combination of different wavelength light sources), but a pure low wavelength light. Why does _violet_ also appear to interpolate between red and blue? Well, Photopsin I has its absorption maximum around green - but it happens to have a smaller absorption peak in low wavelengths, around 400nm as well. So very low wavelength blue light is absorbed by both Photopsin I AND Photopsin III: and your brain deduces incorrectly that your retina is illuminated by some combination of blue and red. And that's why violet looks similar to purple, and the color wheel appears to wrap around as wavelength decreases.

So we have four different kinds of light receptors in the eye: three "cones" that pick up a relatively narrow range, with peaks at the wavelengths we call "red", "green", and "blue", and one kind of "rod" that picks up a wider range of wavelengths that tends to kick in more in low light conditions, and gives you more bright/dark sensations than color.

The color we call "yellow" is what you get when the red and green cones are reporting fairly equal amounts of stimulus. It generally corresponds to the wavelengths between them. Similarly, "cyan" or "light blue" is the green and blue cones reporting fairly similar amounts of light. That color is also between the peaks of those cone's responses.

And then there is "magenta" or "purple". Which is what you get when the red and blue cones are reporting light. This does not correspond to any particular wavelength of light. But inside your brain it still produces a sensation of color, that has similarities to the red and blue experiences.

This is why some people like to say that "purple is not a real color": it does not correspond to a single particular wavelength of light. But really "color" is just something your brain makes up to classify different combinations of excitement of the rods and cones in your eyes anyway; the reality is that our eyes are only sensitive to a tiny fragment of the electromagnetic spectrum, with an uneven distribution - the "red" and "green" cones have a lot of overlap in their sensitivity, while "blue" barely overlaps either. Have a look at the diagrams in Wikipedia's page on "trichromacy": https://en.wikipedia.org/wiki/Trichromacy

This is also why all our displays are based on red, green, and blue lights: you can fool the brain into sensing a particular color by showing the eyes nothing but the three frequencies the cones are most sensitive to, in various amounts.

The L-cones that detect red color also are sensitive to violet light. You can see some of that in some of the curves here: https://biology.stackexchange.com/questions/51870/can-red-co...

From what ive gathered (doing interactive light/sound stuff and chatting with folks at CymaSpace) the issue is that its subjective. Synaesthetes are often frustrated with color to pitch mapping that claims it is "inherent" because the root color (this author picked blue because "it seemed settled") is the creators personal synaesthetic correlation. The issue gets complex when you consider what training set makes each synaesthete build the correlations between colors, moods, sounds, etc.

The article its-self is rad in that the circle of fifths is very much a "relative" thing to begin with so we are actually seeing some pretty "flavorful" results- but the search for the "true color of sound" is tough when light is 430-730 trillion hz and audio is 20-20 thousand hz. The factor that you then are dividing or transposing by is arbitrary.

I associate some pitch classes with colors and I think it's because of a toy glockenspiel I had when I was a kid.

I think that is largely true for my personal experience, however, the timbre of the sound also affects the color for me.

I'd put "syntax highlighing" on the accidentals (maybe red and blue depending on whether it's flat and sharp relative to the scale), and leave everything else alone.

You certainly find this kind of thing in numerous beginner books, but I’m skeptical that it’d be of much use for complex scores, or for someone with mastery of sight-reading. We don’t tend to do the same for text, after all. That said, it is easy enough to miss a dynamic marking and plough through a section at the wrong volume. That one might be worthwhile!

Maybe there is, but not for humans!

If we sensed sound like we sense colour we'd only hear the amount of bass/mid/treble not the individual pitches.

But if there's such a thing as a non-linear medium for light, then transmitting two wavelengths through it should do interesting things depending on the ratio of the wavelengths?

Not really. 2 crucial differences: first, it’s the degree of the scale (I, ii, III…) that’s given a colour rather than the note name (C, D, E…), meaning the colouring is independent of the key (or octave) it is written in or transposed into. Secondly, the colours used are designed to be similar where their harmonic ‘meaning’ is similar (according to traditional Western music) and vice versa.