This use case for SubD faces is essential. The whole point of SubD (at least for us) is that you have an editable, movable, watertight model you can use to explore concepts and keep editing until you/your client/your internal components are happy. It makes sense of course to be able to colorize/materialize the different chucks of SubD as you would do a final broken up model to help you make design decisions.
This feature is a must, I’ve never encounter any SubD modeling program that didn’t have this, so it never occurred to me that it’s missing in Rhino (not using Rhino SubD here yet).
Maybe McNeel should consider a global change to per subobject attributes, not just SubD faces? This also applies to polygons, surfaces…
Also I hope we can UV unwrap just a sub-selection of a SubD and they separately unwrap other UV islands on other sub-selections. Color would be a handy guide to remind ourselves which part of the model is its own UV island.