If he himself were a user of face powder, perhaps he'd just assume that that's 'what everybody does'.

We could watch for later examples of his assuming that someone's using a cosmetic.

However, I tend to agree with LeeAnne. He fancies himself as a comedian (and Mr Shepherd obligingly laughs on cue in chapter 3), so I've always supposed that the powder is hair powder, and either:

• Admiral Baldwin (as described) is so gormless that he slaps it on without realising that he's bald, or

• Admiral Baldwin actually has more hair left than described.