Frankly, the "yellow" was always a bit odd to me too, was it a very specific wavelength of reflected light? And if it was yellow objects that absorb all wavelengths aside from yellow, why was transmitted yellow light the same? What about orange with a lot of yellow in it?
Well, to me it was always the yellow part of the spectrum, which means it is not a specific wavelength, but a range of wavelengths. I suppose the "impurity" in Hal's ring is perilously close to his own green on the spectrum, but yellow is yellow, orange is orange, and green is green. The ring is a scientific device, and as far as I am aware the yellow impurity has been put there by design (whether the Guardians admit it or not), so it will cover specific wavelengths.
But all this is making the issue too complicated for comics. The ring has a yellow impurity in its design, and therefore will not affect anything coloured yellow. I accepted that explanation when I first read it, and it still makes sense now. The only ones who know exactly how it all works are the Guardians, and, even if they explained it, would we understand it?
Wasn't there an explanation made along the way that the yellow impurity was introduced to prevent the Green Lantern from being
too powerful, or even omnipotent? I quite like that explanation, because it makes sense. As Morgan Edge once implied, everyone is potentially corruptible. A built in weakness for the power ring makes sense.