I just finished watching this interesting video:
It does a very quick explanation of how pink light doesn't exist, and that the concept of pink is our brain's attempt at filling in a gap in the spectrum.
My question is, how does our brain perceive this gap? What frequency is this pink color, if it truly doesn't exist? Is it a frequency greater than violet? Or is it greater than red?
Sorry if this question seems trivial.
Your eye has three types of receptor cells, the sensitivity of each type peaking in different spectral regions. Roughly speaking, there's one that peaks in red, one blue, one green. (It's not quite so clear cut, but your brain is really good at sorting out messes like this!) When you look at a fire engine (assuming it's red) It's mostly the red receptors that are firing. If your fire engine is white (as they are in New Haven, CT), all three receptors are firing. For pink fire engines (Yes, they exist! I saw two last month! If you don't believe me, ask Google.) the red receptor is firing full-on, but the green and blue are on too, but not full-on. Maybe 80% on. Almost white, but biased toward red. Your brain takes that in and calls it pink.
BTW black is no stimulation of any of the receptors. Brown corresponds to very little stimulation of any of the three types, but of what there is, the reds are firing more than the greens or blues. So brown is close to black, but biased toward red.
But it's far from that simple: your brain takes into account the adjacent color before deciding what to labeling the color of the object you are looking at. It is not simply a matter of mixing frequencies!!
Not all colors are monochromatic (pure). The spectrum of colors is really only a spectrum of monochromatic colors, and that's why you can represent it on a line.
For non-monchromatic colors, you'd represent them with a gamut instead of a spectrum: 
Here you'll see pink at x=0.45,y=0.3. The monochromatic colors are along the edge, i.e. the edge is the spectrum. The blue numbers 380-700 nanometers are the wavelengths of the monchromatic colors.
As far as I could understand the video, we don't see the color pink, what we see is white light without the green part. This is so because we can see green light in two receptors in our eyes. The first one place the light we see on the red-green scale, and the second place it on the blue-yellow scale. In this system we measure the green twice (which pose an interesting question of evolutionary advantage).
There are two things which bother me in the video you posted; 1. The video claims that pink is the mix of red and blue light. This is not the same as claiming that we don't see the green light (explanation below). 2. It might be understood from the video that pink is our understanding of the entire non-visible part of the electromagnetic spectrum which is completely wrong. We simply can't see that part of the light.
I have found this tool which allows you to combine colors and see the result. Notice that when you only mix blue and red you get magenta (mentioned in the video), to get a true, hello kitty style of pink you need to add some green. Magenta is a man-made color which is a single type of light. Pink (and red, blue and what ever ``natural color'' we want) is a range of colors and not a single mix.
One last remark. Playing with that tool, I have notice that we can do the same trick with cyan (and blueish color) and yellow (and yellowish colors). I think that what is special in the pink case is only that it is such a common and ``cute'' color which is surprising that this is what we actually see.
In the context of this question it is worth while to read a bit about Edwin H.Land the inventor of the Polaroid camera which was the only way to take instant pictures until the electronic revolution.
He really delved into the differences of frequency versus perception by the human eye.
from the link:Also in this decade (nb 1950s), Land first discovered a two-color system for projecting the entire spectrum of hues with only two colors of projecting light (he later found more specifically that one could achieve the same effect using very narrow bands of 500 nm and 557 nm light).
Here is a pdf document with the effect, by E.Land himself.
