It is often said that, when re-pitching yeast, you should generally go from low- to high-gravity, and dark to light beers. When I repitch, I use trub directly. I understand why using a yeast from a high-gravity fermentation is ill-advised, since the yeast has been stressed, but why go light to dark? Is this just a matter of color? If so, how much darkening would you expect to get when brewing, say, an APA after a stout?
There is no reason...that's an old, disproven myth. I've gone from dark to light many times without any problems. The theory is that as you mention you'd carry over color. There's so little color carried over that it's unnoticeable. For example, I've pitched yeast from a dunkel into a pils and not had it darken the pils.
it's about transferrence of color and to some extent flavor. Even though the slurry is heavily diluted, by 20 times or more, you wouldn't want coffee or caramel notes transferring from your dunkelweizen into your hefeweizen. I depends on the beers, and how much of the previous cake you're reusing.
With brewing on top of the yeast cake, there definitely is some carry over. To avoid it, collect some slurry, let it settle and pour off the beer on top before pitching. The amount of transferrence is then much less, and can be considered a non issue.
If you want to figure out why it's a non issue, consider that you're pitching a dense slurry, say 250ml / 10oz. Of that, let's say half is still beer and the rest is trub/yeast. That's 5oz of beer from the previous batch being repitched into 5 gallons, which is a dilution ratio of about 120 fold. With a very dark stout of 50L that would give you about 0.4L color difference. For more moderate beers, the color change and flavor transferrence is tiny.