Yes, I think I misread the thread, and forgot the category (physical anthropology). Not my main focus.
Though the only thing I would know that comes near it would be from the Bibliotheca Historica by Diodorus Siculus, but I doubt that it mentions these specific groups, I do know it has mention of other groups in this regard.
Its doubtful how reliable most of classical sources are however.
See this search:
the picts physical appearance - Google Book Search