This sounds overtly complicated, when one can instead apply good old racial prejudice.
After all, the Borg do consist of different species. Even after assimilation, the characteristics of those species are quite evident: Cardassians have ridges around their eyes (even if the eyes are artificial) and Bolians have a central ridge on their face (even if this face no longer is blue).
The Cube encountered in "Q Who?" had just finished assimilating a planet. Perhaps the people of that planet were of a skin type that looks pale after assimilation? Or perhaps they were an "easy" species that didn't need all that much nanoprobing and thus didn't look quite so necrotic? That is, they got a treatment similar to Picard's, a "soft touch", and would thus only grow mottled later on.
The other Borg vessels encountered in TNG could have been from the same gaggle that assimilated the planet in system J-25, even though obviously they weren't all the same vessel. This task force would loiter in the vicinity of UFP space for a while even after the failed "BoBW" operation. But each vessel in the task force would also include its share of mottled-skin Drones, enough to explain the "deleted scene" from ST:FC.
This would account for the same trend that Christopher
observes, but much more easily, because the pool of "palefaces" would be a tiny one to begin with. It would quickly get diluted into the greater pool of assorted "zombiefaces", and a few years after the assimilation of the J-25 planet, one would be really hard pressed to find a "paleface" anywhere.
Which is good, because we won't encounter those folks in Star Trek ever again - unlike the case in Christopher
's theory where the pool of cloned Drones should return to full strength at some point.