[avz]

To see an extra color, i.e. to become tetrachromats, we would need a lot more than a dietary change, so this is an entirely new subject barely related to the original post.

To distinguish an extra color we would need a new type of cone cells with a different spectral response to the three types we already have (called L-, M- and S-cones). We could modify a subset of the existing cells or add new cells. If you modify the existing cells, you then need to teach the visual cortex that the signals coming from the modified cones indicate a new color. How would this be done? If you add new cone cells, you then need to add new neural pathway along the optic nerve and plug it into the visual cortex somehow. How would this be done?

On the other hand, the original experiment proposed to alter the availability of different types of vitamin A by dietary changes with the intention of modifying the spectral response of the existing photosensitive cells so that light of previously invisible wavelength would become visible. This does not increase the number of colors, but changes the range of wavelength corresponding to each color for the affected individual.

[wlesieutre]

I haven't read it myself, but it's worth pointing out that research has claimed that there already are some humans with four types of cones.

https://en.wikipedia.org/wiki/Tetrachromacy#Possibility_of_h...