Mark 15:17

ἐνδιδύσκουσιν αὐτὸν πορφύραν. Double acc. both here and Mark 15:19. Cf. Luke 16:19 of Dives. Mt. for πορφύραν has χλαμύδα κοκκίνην, Jn has ἱμάτιον πορφυροῦν. All three mean some bright coloured garment to represent a royal robe; See on John 19:2-3 and cf. the Gospel of Peter iii. 7. There are parallels in the Testaments (Zebulon iv. 10; Benjamin ii. 3); and the behaviour of pirates to their captives, as described by Plutarch (Pomp. 24), is a striking illustration. Several others are quoted by Lagrange.

ἀκάνθινον στέφανον. It is impossible to determine what plant was used for this purpose, and conjectures are very various. But the use of στέφανος instead of διάδημα does not prove that the soldiers mock Him as conqueror rather than as king. The whole context indicates mock homage to royalty.