How are there more colours than we can see?

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For example, [this colour wheel has 12 sections](https://www.spectrumnoir.com/wp-content/uploads/2017/04/Colour-Wheel-2.jpg) and performs a full ‘loop’ for lack of a better term (i.e. starts at red, traverses to the polar opposite, and returns the natural way). We can see every one of these clearly, and every colour in between those two colours as it’s simple a mixture, or a mixture of a mixture, or so on. So how can a colour exist outside this wheel?

In: Biology

Our eyes detect color through the number of cone cells we have. They’re photoreceptors, they capture wavelengths and we infer color from that. But we can only see as much as our eyes will let us. Other creatures (like the mantis shrimp for instance) have far more cone cells and therefore a wider range of possible colors to infer.

This color wheel was invented by us, based on only what we can see; it’s not something that taught us all the colors in existence.

Because our eyes have 3 color receptors – red, green and blue. While those receptors, when combined, can perceive millions of colors they don’t always pick up extremely subtle color shifts.

If you want to enjoy something weird about color try imagining bluish yellow, or yellowish blue. You can’t. There are lots of videos on YouTube about blue-yellow, a pretty fun hole to jump down on a boring day.

Humans only have three color cones in the retinas in our eyes. Red, green, blue. Our world is made up of more color than what we can actually see. We cannot comprehend more colors or what they could possibly be because our brains/eyes can never calculate them.

When light strikes a cone, it interacts with a visual pigment which consists of a protein called opsin and a small molecule called a chromophore which in humans is a derivative of vitamin A. Three different kinds of opsins respond to short, medium and long wavelengths of light and lead to the three response curves shown above. For a person to see an object in color, at least two kinds of cones must be triggered, and the perceived color is based on the relative level of excitation of the different cones.

The mantis shrimp has between 12-16 color cones. It’s insane to me that so many colors are out there to see.

There are two things to consider here – what color represents and how we see color.

Color represents how we see different frequencies of light waves. Red has the lowest frequency (that we can see) and violet has the highest. The rainbow ROYGBIV has all of those frequencies, but also extends to frequencies lower than red (infrared means less than red) and higher than violet (ultraviolet means more than violet). These are the colors we can’t see.

Now, onto the color wheel and how we see color. Your eyes have 3 color sensors… red, green, and blue. You can, in effect, “trick” your eye into seeing yellow by mixing red and green (since the frequency for yellow is in between red and green, and your eye doesn’t have a separate yellow sensor). And by mixing red and green, you can get anything from red to orange to yellow to green. This is how your color TV or phone display works – it has red, green, and blue lights and can show millions of colors just by mixing those three. Now onto the color wheel… if you track the rainbow, it goes from RoyGBiv (violet is really just a deep blue), which basically goes from pure red, to red-green mix, to green, to green-blue mix. There is no purple (or red-violet in your color wheel) in the rainbow. That’s because there is no frequency for this color, it is made up by your eye/brain to bridge the gap between blue/violet and red. This is why the wheel seems continuous.

That wheel only shows a single variable, hue, as a function of angle. Depending on which colour model you want to use, it needs three coordinates, Red, Green & Blue or Hue, Lightness & Saturation. Where on that ring is brown? or grey? or sky blue? Your colour ring is too simplistic.

There aren’t.

(Assuming you were just using that image as a general example of a colour wheel. Cuz we can definitely see more colours than those on that wheel literally.)

Colours do not exist out there in the world. Colours only exist in your eyes/brain. There are more *wavelengths* of light out there than humans can detect. But colour is NOT an inherent property of light. Colours are created in our minds.

Other answers are talking about our eyes having only three colour dectection cells. And yes, you see red when ~680 nm light hits your eyes. However, you ALSO see red when light of all wavelengths except ~510 nm hits your eyes (i.e. red = white – green). Colours are not inherent to the light frequency, but instead they are made by how our eyes and brain react and interpret the light falling on our eyes. Pink doesn’t even have an associated wavelength of light. You can *only* see it when you have light of all wavelengths but ~580 nm hitting your eyes.

Physically our eyes only react to wavelengths of light in the 400 to 700 nm range. Though of course this isn’t a hard limit to wavelengths. Light can have higher or lower wavelengths. We don’t detect all of them because it wouldn’t be useful: the atmostphere blocks out most other wavelengths before the light reaches the ground.