The rainbow made by a prism is has 7 colours (red, orange, yellow, green, blue, indigo and violet). Though they seem to somehow merge together these colours appear to be distinct to most people (except for maybe the last 3), ignoring colour-blind people.
Thinking about this I concluded that human eyes must have 4 different colour receptors. Each receptor must receive a small range of frequencies in the electromagnetic spectrum and it seemed likely the ranges overlapped (since there are no invisble gaps in a rainbow). Looking at how the receptors' frequency ranges might overlap it occurred to me that, in general, given N receptors you can detect (2N - 1) different colours (though for certain configurations you might get less). My conclusion was that humans have 4 different types of colour receptors since we see 7 (= 2 x 4 - 1) different colours.
I remember specifically asking my science teacher (Mr Connell?) around about 1975 "why the rainbow has 7 colours", not so much to find out the answer but to find out if it was generally known what I had concluded. His answer was vaguely along the lines of it was to do with physiology and we were studying physics not biology. I actually did a bit of research but I could find no information about this (the Internet did not exist at the time).
It wasn't until several years later (doing Psychology I at Sydney University) that I found out that the eye has rod and cone cells to detect light. There are 3 different types of cone cells used for the detection of different colours, while rod cells are used for "night vision".
I am still waiting to hear of research that shows that rod cells also play a part in colour reception (probably in the blue-indigo-violet region). Then there would be the 4 colour receptors I had theorized.
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