Assigning Materials to Parametric Cells

The new Parametric cells when placed in a design, don't seem to be able to accept byLevel:Colour material assignments. This may have something to do with the cells definition being a bit like a shared cell. And the instance sitting on the default level???

If I pre-define materials, and pre-assign them in the cell itself, then materials are respected when placed in a design - but this workflow isn't all that useful if I want to test out a couple of different materials in the placed design.

Am I missing something obvious?