So colors between deep blue and purple-red are not supposed to exist. QI did not explain why. I've seen in some textbooks that these colors are described as mysterious or anomolous. In the textbooks this diagram is used...
...which at least gives a bit more "explanation" of why they should not be visible. According to some people, since they are on that strange lower edge, and not on the "spectral edge" they are therefore non-existent colors.
(I found the reasoning to have the same weight as those who say "science says bumble bees should not be able to fly, but they can, so science is wrong!" It is clear to anyone with half a brain that bumble bees are not shaped like aircraft, are lighter and less dense. The science which explains bumble bee flight is going to be different to the science which explains the flight of huge passenger jets.)
Back to colors though. There's two things wrong with this "science says we can't see mysterious colors but we can!" "reasoning":
- We can see colors which are not on the spectral edge, white is a good example. In other words we can see colors which are inside the shape shown above. Light yellow is another good example. So how near to the mysterious edge does a color need to be for it to become become mysterious? Clearly an arbitrary distance.
- We see colors because our eyes/brain (during daylight) mixes singals from three sensors (red green and blue). Our eyes don't even "know" about that mysterious edge. The diagram above is useful but it is not what goes on in the brain.