Why does the chromaticity diagram look like that?

I think the explanation is simple: Color is light and it is linear going from ultraviolet to blue to green to yellow to red to infrared. It's just a line.

In physical reality, there exists no purple light. Our minds make up all the shades of purple and magenta between blue and red when our eyes receive both red and blue light.

So in order to include the magentas, you need to draw another line between blue and red. Meaning you have to bend the real color line. And that's what we see in the chromaticity diagram.

Wavelength (or frequency) is linear but light, in general, is made up of many wavelengths -- an entire spectrum.

Each wavelength of visible light corresponds to a color on the gradient from blue-green-yellow-red. Purple or magenta colors do not exist as light and only exists in our minds. That's why rainbows do not contain any of these colors.

Purple totally exists, but isn’t a single wavelength of light. It is multiple wavelengths of light. Physical colors are all blends of wavelengths.

Displays are tricking the eye by showing three single colors that look like real color.

> Color is light

For an ELI5 on a "maybe teach color better by emphasizing spectra?" side project, I went for hard disjointness on "color" vs "light". Distinguishing world-physics-light from wetware-perception-color. Writing not "red light", but "\"red\" light". So physical spectra were grayscale, on nm and energy. Paired with perceptual spectra in color, on hue angle and luminosity. And both could be wrapped around a 3D perceptual color space (tweening the physical spectra from nm to hue). Or along a 2D non-primate mammalian dichromat space, to emphasize the wetware dependence. Misconceptions around color are so very pervasive, K-graduate, that extreme care for clarity seems helpful.

As a hue, magenta and purple shades do not physically exist in the electromagnetic spectrum. All hues on the gradient blue-green-yellow-red exist and can be generated by a single wavelength of radiation.

You can test this in physical reality with a prism, which will never show purple shades, because it is an extraspectral color that is made up in our minds.

Color can thus exist as pure in physical reality. However, our eyes can maybe not perceive colors purely, since our receptors overlap each other.

Colors are not single wavelengths. If you are redefining what color means, you should use a different word to reduce confusion. Maybe spectral color.

Secondary colors are colors. Notice that the hue on color wheel includes magenta and purple because it includes mixtures of the primary. Magenta and purple exist on electromagnetic spectrum but not as single wavelengths.

There are imaginary colors that are represented in color space but not by physical light spectrum. But purple and magenta are not imaginary. As can tell from the Roman emperors' clothing.

The discussion on this specific graph is fairly scientific and not about cultural color perception. I changed the wording to "hue" to be more clear. Magenta and purple absolutely do not exist on the electromagnetic spectrum as their own wavelengths. Every other hue of other colors we can perceive does.

"Primary" and "secondary" colors are not scientific terms, but cultural terms.

The evidence is right there in physical reality, a rainbow or a prism will not include magenta/purple, because the colors between red and blue are not part of the spectrum. It is an amazing thing that we can make up these shades in ur minds.

But I was wrong in my original comment, because the red-blue connection can also be done by making a color wheel, ie making a complete curve.

Edit: So let's make a comparison with sound, which is also waves, but not radiation. Every note of music corresponds to a pure sine wave. Just as every color hue corresponds to a pure electromagnetic wave. Except the purple and magentas, who do not occur like this.

Then you can say that most colors we see are mixed and reflected, which is true, just as most notes are not pure sine waves.

> I think the explanation is simple: Color is light and it is linear going from ultraviolet to blue to green to yellow to red to infrared. It's just a line.

The most common misunderstanding of color is that it has any property extrinsic to the seer. Color is in the mind.

> In physical reality, there exists no purple light.

In what you imply is physical reality (optics) there's no color at all.

> Our minds make up all the shades of purple and magenta between blue and red when our eyes receive both red and blue light.

To repeat, your mind makes up every color. It's a category error to imbue light with a trait of color independent of the seer. In color science, what you refer to as physical reality is more precisely termed spectral power distribution of electromagnetic radiation in the visible range.

To the extent that radiation stimulates a color response for the human visual system, there most certainly is purple light, it's just not simulated by a single narrow band of radiation.

As you have attempted to note, The CIE horseshoe diagram (spectrum locus) illustrates this with the "purple boundary" at the bottom, which represents the perceptual edge of mixing long and short wavelength stimulus (red/blue primaries).

But to presume that there's a natural color of light is a distinctly human biased statement of reality. Different creatures have characteristic responses, with very different traits from humans. Imagine having sensitivity to polarization.

The artist's color wheel is circular because that's how color as qualia manifests in the mind of the artist. The physics of the qualia are more with complex intrinsic response of the organism than the extrinsic stimulus.

To speak of the color of light is common sense, but over-imbues the physics of the stimulus with human traits.

Make explanations as simple as possible but no simpler.

> ...Meaning you have to bend the real color line. And that's what we see in the chromaticity diagram.

Any explanation that regards "real color" as a trait of light rather than of perception misleads more than it clarifies.

For example infrared and ultraviolet wavelengths have many practical uses that proves that they exist in physical reality, even though we cannot even see those colors. That should be enough evidence for the person who is interested in truth.

Sound waves also exist in physical reality and makes a medium vibrate in correspondence to what we hear. But your argument would be that sound does not exist either and is only made up in our mind? Maybe everything is made up in our minds then, and we arrive at the most boring and meaningless philosophical discussions that have ever existed.

But regarding my first comment, I've written elsewhere below that it was incorrect. As the spectrum - as perceived by us - can also be presented as a circle or other shapes.

> To the extent that radiation stimulates a color response for the human visual system, there most certainly is purple light, it's just not simulated by a single narrow band of radiation.

How colors are perceived maybe depends on the perceiver. But not in the way you argue above. It is evident that purple/magenta light does not have any corresponding wave length in the electromagnetic spectrum that we can perceive and that the only way we perceive it is when mixing blue and red light. The conclusion is that these colors are made in our minds. Unless you can provide an example of a wave length that would be seen as purple by anybody?

Color is the way our eyes and mind interpret different wavelengths of light. Will you agree with this?