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
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.
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