At Thes. 17 Plutarch states that, according to Simonides, Theseus’ ship was steered by the Trojan Phereclus, and in case of success it was to raise not a white sail, but a purple one, colored with the dye obtained from the holm-oak (pace Orlando Poltera). Through both these details the poet emphasized the Athenians’ sailing inexperience at that early time, when they still used the ‘terrestrial’ dye rather than the Phoenician purple, extracted from a sea shell. This paper dwells on this dye’s peculiarities, by referring to modern scientific explanations as well as to the several ancient opinions on the matter.