I imagine that he is equating the spectrum back to the rainbow, where red is the ‘top’ of the visible arch when a single rainbow is seen from the ground.
Of course the outer ring from a double rainbow flips the order of the colours, but that’s probably being needlessly pedantic.
It’s percentage of the entire population, if you organized everyone from lowest to highest IQ, what percentage of the entire population would you need to include for them to be in the top. It’s a little clearer when a point of interest is near the top. E.G. “person is in the top 1% of people in terms of wealth.”
A less kind way to say it is ‘91% of people have a higher IQ than this person’.