Kuhli loaches don't really like the light so they tend to be "nocturnal" (not truly a nocturnal species IMHO though), usually only coming out and being active at night.
Fish can see the color differences in substrate and all things, but I doubt the kuhli loaches will really care. If they had a choice of where their "cave" would be, it would be on darker substrate/area (since they blend in and feel safer on darker substrates), but that doesn't mean they won't slither/venture out onto the light colored substrate.
I keep at least 10 different Kuhli loaches species (Pangio species) and none of mine have never burrowed (even if frightened when you are trying to catch them and no where to run/hide). I have read that they do burrow, but in my experience and seeing my LFS employees struggle to net the kuhlis, I find this false. I do keep many other loaches species that do burrow and so I am familiar with burrowing, which in my experience Kuhli's do not. Also my kuhli's have never been destructive, even if they are zooming around, even if startled and they dart away bumping into rather small, lightly rooted plants, they haven't done any damage or practically don't even stir up any fine sand substrate at all. And I do have Giant Kuhli loaches (Pangio Myersi) as well, so if any kuhli's would uproot anything, it would be those big boys, but they haven't, no harm even with them sifting all around the plants.
But as far as a Zen vibe, kuhli's do look like they swim/dance chaotically against the glass. They are extremely peaceful, but their swimming behavior isn't exactly what I would call graceful. But they can/do swim in circle against the glass which is sort of "Zen"-like. You know what would be cool though, if in your Zen garden, you had a Pagoda décor there that the kuhli's lived in with their little heads poking out haha.
Oh, just re-read your thread, the rock "garden" on the white sand, I bet some kuhli's would even live in that rock pile if there isn't more "shaded" caves.
But again, just to recap, the fish will have no problem crossing the "color boundary". So your plan is perfectly fine, just give the kuhli's plenty of dark hiding places.
A tip is, if you provide plenty of thick carpet plants (just something they can crawl through feel safe in the cover of the foliage, even lots of pygmy chain swords will do), the kuhli's are more happy to venture through the foreground during the day.
Enjoy the Kuhli's!