[EventH-]

Not sure what the justification is for saying Y functions 'psychologically' as a primary color. It would be a secondary color (R + G) in terms of the light spectrum, and (as you noted) our cones are directly most sensitive to RGB.

To answer the question, as qualia cannot be compared from person to person, there is in principle no way to show that they differ (or are the same) objectively. A sort of relative comparison of qualia within an individual (as you propose) will also not get you there, as it may be that the individual is simply more sensitive to changes along the gradient, not that they are experiencing a new color, and that's assuming we are able to use an objective comparison scale which we aren't.

[crazygringo]

See the link I included on the opponent process. It explains how we have neurons that respond specifically to red/green and yellow/blue. That's why Y is psychologically as primary as the other 3.

Here's another way of looking at it: psychologically, we don't perceive yellow as a mixture of red and green, the way we perceive purple as a mixture of red and blue. Yellow isn't "perceived" as a mixture of anything in our minds -- it's perceived as primary. If you say "it's kind of a reddish-green", nobody is going to think, "oh you mean yellow!" While orange is perceived as a mixture of red and yellow, for example.

Remember, I'm not talking about what's happening physically with wavelengths -- I'm talking about psychological perception of colors.

[OscarCunningham]

> our cones are directly most sensitive to RGB.

The L cone is actually most sensitive to the yellow wavelengths.

[EventH-]

You're right, thanks.