I think this is a very good explanation. You're emphasizing that a trichromat will experience the same activation of their cones to the wavelength of yellow light, and light containing the wavelengths of red and green mixed. So the spectra of these two colors will be different, but appear identical.

Whereas a hypothetical tetrachromat with an additional cone type that activates to the yellow wavelength, will have obviously different activations of their cones for the "wavelength of yellow" and "wavelengths of red and green mixed". And hence experience them as different colors.

I'm not familiar enough with the different definitions of color spaces to define how to model this. You could easily define it as a four-dimensional RGBY color space, but to my understanding the other color spaces are defined because they have better properties for combining different colors, or playing better with interpolation between colors.

It would be a challenge to come up with a good four-dimensional color space that is a useful artistic and visual tool, given that there are very few people around to evaluate it! And also because all computer monitors are trichromatic, so it'd be a job in itself to set up a test system.

I wish someone in the interview had asked some of these people how they experience photos on computers!

You can arrange the three colors in a triangle, which defines a plane, is treated like a circle in color theory, and looks like a weird stretched out "U" when put in a perceptually uniform space (stretched towards green, which we see best, and flat on the magenta side). Perceptual color spaces combine this with luminance, which is sensed by rods instead of cones.

Tetrachromatic colors could be placed in a tetrahedron, defining a space and being approximated as a sphere. A perceptually uniform representation would probably be curved on the three edges that agree with the spectrum and flat on the three that don't. Green is still probably the biggest. Then you'd add luminance as a fourth dimension because it's still rods.

Great explanation, thanks :)

Interesting idea, however I think this is too simple. With three cones we have a three-dimensional space of color. We can express this space with RGB or HSV and so on, just by a three dimensional basis vector we can describe one color point in the color space.

With tetrachromats we get a four-dimensional space of color. So we need something like RGBX or a two-dimensional hue value like for example H1,H2,S,V. That's what I meant with a second hue circle orthogonal to the one we already know.

What we see as yellow is for the tetrachromat a whole second hue circle varying in the value of second hue.

An example: Yellow is a point on the first hue circle. For the tetrachromat these colors are different, but boring yellow for us:

- pure yellow

- mixture of red and green

- mixture of pure yellow, red and green

In other words, what we see as yellow is a whole dimension of different colors for tetrachromats.

They also would probably be able to distinguish yellow+blue from pure green.

Also, if you mix R+B, that's magenta, and if you start mixing in blue it will turn white or gray. But if you can see yellow, RYB would look different from R+a little g+B, which is what RYB would look for trichromats, I think.