The Ultima VI Underworld is now mostly constructed, connected, roughly colored, and textured, and now I can just paint the caverns with the Metapaint layer, like I can with the overworld, to give each a unique feel. The colors are not final of course, and each cavern will have its own wall and floor stone textures, etc, but this is an early way to delineate different vibes for different regions.
Here's the original fb UDIC post.