> Hypothetical tetrachromacy has been demonstrated, that is, M and L cones with minor differences in peak spectral response existing in a retinal mosaic. The question arises whether this difference is capable of being propagated through the bipolar connecting neuronal paths (which form the "blue-yellow" and "red-green" channel paths conveying color info to the brain visual cortex).

Adding genes for extra cone types into normally dichromatic mammals (mice or something? I don’t remember off hand and I don’t have the citation at my fingertips) has seemed in studies to result in trichromatic vision, so it’s not too big a leap to suspect that tetrachromacy of the type described in the article might happen for some humans.

As you say there hasn’t been any study which showed this conclusively (that I’ve seen anyhow). I’d love to see more thorough research on these subjects who supposedly possess such vision.

> In art, there are great colorists and those who are virtually incapable of subtle use of color. If anyone interested in finding out about their own abilities, check out: http://www.xrite.com/online-color-test-challenge

Note, this online test is a bit easier than the paper version of the Farnsworth–Munsell hue test, since the colors in the screen version end up with some lightness differences that make it a bit easier to keep some sections of the chart in order. Either way, the test IMO mostly measures (a) whether someone has “normal” trichromatic vision, and (b) how patient and willing to fiddle with fine details they are. I don’t personally think Farnsworth–Munsell test scores are super meaningful, though a very poor score does indicate some color vision deficiency. [FWIW, if I take the time to do the test slowly, either on screen or on paper, I consistently score ~0, occasionally mixing up one pair or another.]

But anyway, I think the ability to be a good colorist in art has a whole lot to do with practice. Spending a lot of time mixing paint or color correcting photographs is likely to heighten awareness of tiny distinctions. For instance, I know that after a couple years of photography courses, several of my friends got much better at noticing color casts in photographs. Not that they couldn’t physically see them before, but after experience they more often spontaneously noticed the casts and started to have a feel for just how much adjustment in which direction would be necessary to counteract them.