If colour is a spectrum of frequencies from low to high, why does it form a circle? And why can we make it all up out of three 'primary' frequencies?
I never quite got this, and this article on Pitch and Color Recognition is a fascinating read.
In short, our eyes have specific receptors ('cones') for red, green, and blue light. But they don't just pick up one particular frequency from the spectrum, they respond strongly to a small range of frequencies, with a bell-curve decay as the colour changes.
The odd-one-out is the red receptor, which starts to trigger again at low frequencies (violet). The perceived 'hue' is determined by the ratio of the responses, which is similar at low and high frequencies, thus the cycle.
Now if you work your way through the spectrum, measuring the response to pure monochromatic light of each colour, you get the blue horseshoe path in this diagram which I had often seen but never understood.
X is full red response with nothing else, Y is full green response, and Z is full blue response. Note that no pure frequency of light can get this response - there's no point on the previous graph where only one receptor responds.
If you mix two colours together, the responses get added together (that is, averaged) so the response lies inside the horseshoe. So the horseshoe contains all the colours that we could observe in the physical world. What do the points outside, such as X, Y, and Z represent? There are 'colours', which we are biologically capable of perceiving, that don't exist in the real world.
As for the RGB colour triangle - it's an approximation. Mixing light from the three points labeled R, G, and B can achieve any colour inside the triangle they form. The small area outside the triangle but inside the horseshoe contains colours that occur (rarely) in nature, and cannot be accurately displayed on a monitor or television. Adding more primary colours is limited help - they have to be actual frequencies of light, so they lie on the horseshoe and approximate it with a polygon. You can never represent all colours by mixing a finite number together, although you can get arbitrarily close.
There's so much more in the article though...
