It's not so much a matter of being able to distinguish one wavelength from another as it is being able to distinguish one wavelength from a combination of other

It's not so much a matter of being able to distinguish one wavelength from another as it is being able to distinguish one wavelength from a combination of other wavelengths, or one combination for another. For example, monochromatic orange light falls between about 590 and 620nm. But if you get a beam of red at half[1] the intensity of the orange mixed with a beam of yellow/green at half the intensity of the orange, you won't be able to tell the difference between that combo and actual monochromatic orange given a simple RGB array. [1] Substitute "half" with whatever the appropriate scaling function is for the sensors and wavelength ranges in question.