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

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.