Just a guess, but it sounds to me like he might be describing light/dark adaptation? I remember noticing this as a kid, particularly when one eye got adapted differently than the other (e.g. lying on my side in the grass on a sunny afternoon, so that one eye was in shade and the other in sun). Not only did the world look darker through one eye (the one that had been in the sun) but the color spectrum looked different.
Another possibility is that it might be mild "color blindness." He might be a partial dichromat (I don't know if such a thing has been documented, but it seems like it could easily be possible), or an anomalous trichromat (those people have all three color receptor types, but one of them responds to a different wavelength of light than normal).
But I think it's informative that he says he "sometimes" sees this way. That suggests that something changes (in other words, it's a transient effect). If he saw the world that way all the time, he wouldn't really know to describe it that way. He would just think that's the way everyone saw it.
