The base was modelled after the base of a tabby cat, so it's gray with those taupe undertones. Though it does look close to Shale, a type of rock! The base itself leans close to Sidereal but with more depth and no speckles.
If it doesn't end up uncommon, it could be a cool combo base. (Gunmetal/Soot/Platinum X Haruspex/Harbinger/Seer/Penumbra ) As a way to give use to the September and November bases