[angstrom]

Wouldn't be surprised if it's similar to the coatings placed on the launch pad stabilizers. The paint is meant to char because it acts similar to a ceramic preventing damage to the underlying metal. There's a good youtube video of this on the moon launches where the tail fin hold downs char, but don't warp.

[greglindahl]

The grid fins have paint like that, which is why they catch on fire.

[dflock]

They're aluminium underneath the paint - and that also melts a bit on a high speed GTO reentry, hence the likely switch to titanium grid fins in the future.