This looks great. Object colours as default would be my preference, but as you say, an option in the STEP export dialogue could allow users to choose.
What prevents Rhino from reading multiple colours in a single polysurface when importing a STEP?
Do the big-name solid modellers all handle colours in the same way when importing STEP files?
On a different (but related) tack, it would be VERY useful if there was an option to attach Property Names to parts when exporting to STEP, such that solid modellers have them appear in their model tree on import. At the moment, the only reliable way I’ve found to do this is by making each Rhino polysurface a block, but this is both time consuming and restrictive when one needs to carry on working with the Rhino model afterwards. I know that the name tags are currently exported, as I’ve had success in sending models to someone using an old (UNIX) seat of Catia (without using blocks), so a lot of the blame should be put at the feet of the ‘big boys’ and their lazy approach to non-native file import. Not for the first time, either…
Anyway, thanks for the work on multicolour polysurfaces, irrespective of whether they can be saved to STEP or not. It’s a very useful feature. Oh, and please make sure this works when saving out a 3D PDF!