there is a school of thought that the word Prythain (and the Q-Celtic version Cruithne) existed prior to the Greeks using Pretani to describe what is now Britain, the meaning of which isn't known. the Greeks may then have heard this name and because it is similar to their word for "painted ones" started calling Britain the Pretani isles, which would also seem appropriate to them since the "Britons" apparently tattooed themselves or used woad

Cheers
Andy