With purple - Matthew says scarlet. See the notes at Matthew 27:28.
About his head - In the form of a garland or diadem. The whole head was not covered, but it was placed in a circle round the temples.

And platted a crown of thorns - In the note on Matthew 27:29 (note), I have ventured to express a doubt whether our Lord was crowned with thorns, in our sense of the word; this crown being designed as an instrument of torture. I am still of the same opinion, having considered the subject more closely since writing that note. As there I have referred to Bishop Pearce, a man whose merit as a commentator is far beyond my praise, and who, it is to be regretted, did not complete his work on the New Testament, I think it right to insert the whole of his note here.
"The word ακανθων may as well be the plural genitive case of the word ακανθος as of ακανθη: if of the latter, it is rightly translated, of thorns; but the former would signify what we call bear's-foot, and the French, branche ursine. This is not of the thorny kind of plants, but is soft and smooth. Virgil calls it mollis acanthus, Ecl. iii. 45, Geor. iv. 137. So does Pliny, sec. Epist. ver. 6. And Pliny the elder, in his Nat. Hist. xxii. 22, p. 277, edit. Hard., says that it is laevis, smooth; and that it is one of those plants that are cultivated in gardens. I have somewhere read, but cannot at present recollect where, that this soft and smooth herb was very common in and about Jerusalem. I find nothing in the New Testament said concerning this crown, which Pilate's soldiers put on the head of Jesus, to incline one to think that it was of thorns, and intended, as is usually supposed, to put him to pain. The reed put into his hand, and the scarlet robe on his back, were only meant as marks of mockery and contempt. One may also reasonably judge, by the soldiers being said to plat this crown, that it was not composed of such twigs and leaves as were of a thorny nature. I do not find that it is mentioned by any of the primitive Christian writers as an instance of the cruelty used towards our Savior, before he was led to his crucifixion, till the time of Tertullian, who lived after Jesus's death at the distance of above 160 years. He indeed seems to have understood ακανθων in the sense of thorns, and says, De Corona Militar. sect. xiv. edit. Pamel. Franck. 1597, Quale, oro te, Jesus Christus sertum pro utroque sexu subiit? Ex spinis, opinor, et tribulis. The total silence of Polycarp, Barnabas, Clem. Romanus, and all the other Christian writers whose works are now extant, and who wrote before Tertullian, in particular, will give some weight to incline one to think that this crown was not platted with thorns. But as this is a point on which we have not sufficient evidence, I leave it almost in the same state of uncertainty in which I found it. The reader may see a satisfactory account of acanthus, bear's-foot, in Quincy's English Dispensatory, part ii. sect. 3, edit. 8, 1742."
This is the whole of the learned and judicious prelate's note; on which I have only to observed that the species of acanthus described by Virgil and the two Plinys, as mollis and laevis, soft and smooth, is, no doubt, the same as that formerly used in medicine, and described by Quincy and other pharmacopaeists; but there are other species of the same plant that are prickly, and particularly those called the acanthus spinosus, and the ilicifolius, the latter of which is common in both the Indies: this has leaves something like our common holly, the jagged edges of which are armed with prickles; but I do not conceive that this kind was used, nor indeed any other plant of a thorny nature, as the Roman soldiers who platted the crown could have no interest in adding to our Lord's sufferings; though they smote him with the rod, yet their chief object was to render him ridiculous, for pretending, as they imagined, to regal authority. The common wild acanthas or bear's-foot, which I have often met in the dry turf bogs in Ireland, though it have the appearance of being prickly, yet is not, in fact, so. Several shoots grow from one root, about four or five inches long, and about as thick as a little finger. A parcel of such branches, platted by their roots in a string, night be made to look even ornamental, tied about the temples and round the head. It would finely imitate a crown or diadem. But I know not if this plant be a native of Judea.

(2) And they clothed him with purple, and platted a crown of thorns, and put it about his [head],
(2) Christ going about to take away the sins of men, who went about to usurp the throne of God himself, is condemned as one that sought diligently after the kingdom, and mocked with a false show of a kingdom, that we on the other hand, who will indeed be eternal kings, might receive the crowns of glory from God's own hand.

And they clothed him with purple,.... Matthew calls it a "scarlet" robe; and the Persic version here renders it a "red garment": it was of a colour resembling purple; it was pretty near it, and therefore so called; which is what kings were used to wear; and so in derision of him, as a king, clothed him with this mock purple robe; and which was very likely one of the soldiers' old coats:
and platted a crown of thorns, and put it about his head; for a crown, and also a reed in his hand, instead of a sceptre, as Matthew relates; See Gill on Matthew 26:28, Matthew 26:29.

Purple - As royal robes were usually purple and scarlet, St. Mark and John term this a purple robe, St. Matthew a scarlet one. The Tyrian purple is said not to have been very different from scarlet.