OK, I got it working but I’m not sure why.
First I set an unused cache key on the onSketchfabUtilityReady function:
// other actual textures assigned subsequent cache keys
Then I called that cache key to remove the non existent texture before setting the colour:
After that I can toggle back and forth between various textures and colours, but not if a setMaterialUIDPending cache key has not been removed.
Is this the Utility way of setting the texture channel to null, or am I totally off??