Thanks Igor & Pilou & unc for the additional feedback, that's helping to get things on a clearer track.

This whole business of "SketchUp has layers but you shouldn't use them" is kind of weird.

Also the interaction in SketchUp between groups and layers was somewhat baffling to me for a bit, but I think I've figured out now that layer0 is treated in a special way, with geometry set to layer0 taking on the color of the group's own layer, while geometry in other layers retains its geometry's own layer color or something along those lines.

The whole idea of a group having its own layer assignment as a separate thing from its geometry's layer assignments just seems like a kind of strange thing, particularly with the idea of overriding just some of those children's layer assignments but not others...

Some of this appears to apply to materials as well, since there seems to be a concept that a group can have its own material assignment as a separate thing from the materials assigned to faces within the group. It looks like if the faces are assigned to their own materials, then assigning a material to the group (or using the paint bucket on the whole group object) does nothing at all, which seems like it may be confusing to people.

On the other hand if I assign materials only to groups then that doesn't seem like it will be compatible with multiple materials within the same solid (if solids are exported as groups), like if you assign the top face of a box in MoI to be Style = Red, but have the side walls be Style = Blue...

I guess that I would assign materials to the faces so that multi-material solids would work as expected, and then just hope that people will know that they need to double-click or explode the group before their paintbrush tool in SketchUp would actually do anything to it. These kinds of things may be why I would not export objects as groups by default but have it as an option that you could turn on.

- Michael