Red Hair and the Writer Tacitus

I've recently found a bevy of quotes about red hair from the Roman historian and senator Cornelius Tacitus. They come from three separate works.

Firstly, from The Life of Agricola, this description of the ancient Britons;

Who were the original inhabitants of Britain, whether they were indigenous or foreign, is, as usual among barbarians, little known. Their physical characteristics are various and from these conclusions may be drawn. The red hair and large limbs of the inhabitants of Caledonia point clearly to a German origin. The dark complexion of the Silures, their usually curly hair, and the fact that Spain is the opposite shore to them, are an evidence that Iberians of a former date crossed over and occupied these parts. Those who are nearest to the Gauls are also like them, either from the permanent influence of original descent, or, because in countries which run out so far to meet each other, climate has produced similar physical qualities.

Then this from Germany and its Tribes;

For my own part, I agree with those who think that the tribes of Germany are free from all taint of inter-marriages with foreign nations, and that they appear as a distinct, unmixed race, like none but themselves. Hence, too, the same physical peculiarities throughout so vast a population. All have fierce blue eyes, red hair, huge frames, fit only for a sudden exertion. They are less able to bear laborious work. Heat and thirst they cannot in the least endure; to cold and hunger their climate and their soil inure them.

And finally, this interesting passage about a chap named Civilis from The History;

Then Civilis fulfilled a vow often made by barbarians; his hair, which he had let grow long and coloured with a red dye from the day of taking up arms against Rome, he now cut short, when the destruction of the legions had been accomplished.