I like to envision the Picts as being a cultural and racial mix of the North American Indian and the pre-Roman Celt/Gael-like tribes of the British Isles. I don't think that REH intended for them to perfectly mirror any one modern race, but rather to reflect the practices and appearance of both of these 'savage', and barbaric tribal peoples. It does stand to note that they are referred to as 'white men', but are not regarded as such by the majority of 'civilized' Hyborians, so the Picts swarthy complexion with distinctly caucasian features seems to describe a native American appearance despite their placement in a distinctly EurAsian setting. I'm sure that living in early 20th century Texas, Howards proximity to native American culture (and the old western stories about them) went a long way to influence his development of Picts in the Hyborian Age.