Ceramite -- the stuff that all categories power armour is made out of, and carapace armour to boot. A composite ceramic metal. Very good heat insulator (and therefore resistant to energy weapons).
But: what colour is it before the astartes paint it?
In some of the Horus Heresy novels, I think it is noted as being a kind of grey, dull colour (particularly the loyalists who escaped on board the Eisenstein and removed their previous heraldry). So, is it grey? Much like the standard plastic colour that marines come in? Or something else (silver?).
Does this impact how we might paint battle damage? i.e. should sculpted holes from bolt rounds be painted grey rather than silver as I've been doing for a while? or something else?
Looking forward to hearing your thoughts!