Edwyn Robert Bevan, The House of Seleucus

As we cast round our eyes, we should have observed that while material and colour were of an Oriental splendour, the form was Greek. By the fashion of column and doorway, the painted walls, the shape of candelabrum and cup, the dresses of men and women, we should have known ourselves in a Greek house. The King wore as the symbol of his royalty a band tied about his head. This use of the diadem was Oriental. But here again the form was Greek. The diadem of the Oriental kings was an elaborate head-dress; the diadem of the Greek kings was such as was common in Greece, as a sign, not of royalty, but of victory in the games – a narrow linen band.

The royal dress was the old national dress of the Macedonians glorified. That had not been like the garb worn by Greek citizens in the city, but such as was worn for hunting and riding, and was therefore characteristic of the Northern Greeks and Macedonians, who lived an open country life. It consisted of a broad-brimmed hat, a shawl or mantle brooched at the throat or shoulder and falling on either side to about the knees in “wings” (the chlamys), and high-laced boots with thick soles (κρηπιδες). Of these three parts – the hat, the chlamys, and the high boots – the royal dress of a Ptolemy or a Seleucid king was to the end composed.1 But it was gorgeously transfigured. The peculiar Macedonian hat, the kausia, had apparently no crown; it was a large felt disc attached to the head, and suggested a mushroom to the Athenian mocker.2 As worn by the kings, it was dyed crimson with the precious juice won by immense labour from the sea,3 and the diadem was in some way tied round it, or under it, its ends hanging loose about the neck. The diadem itself was inwrought with golden thread. The chlamys was no less splendid. That made for Demetrius Poliorcetes, when King in Macedonia, is described to us. It was of the darkness of the night-sky, covered with golden stars – all the constellations and signs of the Zodiac. The boots of the same king were of crimson felt, embroidered with gold.4

Edwyn Robert Bevan, The House of Seleucus, Vol. II, Chapter XXXII, p. 274
(Ares Publishers, Chicago, 1985; reprint of the London 1902 original edition)

Ancient sources cited by Bevan:

Plutarch, The Life of Antony, 54.4-6:In the second place, he (Marcus Antonius) proclaimed his own sons by Cleopatra (Kleopatra VII) Kings of Kings, and to Alexander he allotted Armenia, Media and Parthia (when he should have subdued it), to Ptolemy Phoenicia, Syria, and Cilicia. At the same time he also produced his sons, Alexander arrayed in Median garb, which included a tiara and upright head-dress, Ptolemy in boots, short cloak, and broad-brimmed hat surmounted by a diadem. For the latter was the dress of the kings who followed Alexander, the former that of Medes and Armenians. And when the boys had embraced their parents, one was given a bodyguard of Armenians, the other of Macedonians.
(The Loeb Classical Library, translated by Bernadotte Perrin. Electronic source: Plutarch’s Lives on William P. Thayer’s Web Site LacusCurtius.)

Plautus, Trinummus: The Three Pieces of Money, Act IV, Scene 2:
A sharper: To this day I give the name of “The Festival of the Three Pieces” (Trinummus); for, on this day, have I let out my services in a cheating scheme for three pieces of money. I am just arrived from Seleucia, Macedonia, Asia, and Arabia, – places which I never visited either with my eye or with my foot. See now, what business poverty brings upon the man that is wretchedly destitute; inasmuch as I am now obliged, for the sake of three pieces of money, to say that I received these letters from a certain person, about whom I don’t know, nor have I ever known, who the man is, nor do I know this for certain, whether he was ever born or not.
Charmides (behind): Faith, this fellow’s surely of the mushroom genus; he covers himself entirely with his top. The countenance of the fellow appears to be Illyrian; he comes, too, in that garb.
(The Comedies of Plautus. Ed. Henry Thomas Riley. London, G. Bell and Sons, 1912. Electronic source: The Perseus Digital Library – T. Maccius Plautus, Trinummus: The Three Pieces of Money.)

Athenaios of Naukratis, Deipnosophistai, 12:And Duris (Duris of Samos) says, in the twenty-second book of his History,–“... But Demetrius (Demetrios Poliorcetes) outdid them all; for the very shoes which he wore he had made in a most costly manner; for in its form it was a kind of buskin, made of most expensive purple wool; and on this the makers wove a great deal of golden embroidery, both before and behind; and his cloak was of a brilliant tawny colour; and, in short, a representation of the heavens was woven into it, having the stars and twelve signs of the Zodiac all wrought in gold; and his head-band was spangled all over with gold, binding on a purple broad-brimmed hat in such a manner that the outer fringes hung down the back. ...”
(Translated by C. D. Yonge. London, Henry G. Bohn, 1854. Electronic source: The Literature Collection at the University of Wisconsin – The deipnosophists, or, Banquet of the learned of Athenaus, Vol. III.)

Plutarch, Demetrius, 41.3-5:For Pyrrhus (Pyrrhos of Epirus), who was not so much hated for what he had done as he was admired for making most of his conquests in person, acquired from this battle a great and splendid name among the Macedonians, and many of them were moved to say that in him alone of all the kings could they see an image of the great Alexander’s (Alexander the Great) daring; whereas the others, and particularly Demetrius (Demetrios Poliorcetes), did but assume Alexander’s majesty and pomp, like actors on a stage. And there was in truth much of the theatrical about Demetrius, who not only had an extravagant array of cloakings and head-gear – double-mitred broad-brimmed hats and purple robes shot with gold, but also equipped his feet with gold-embroidered shoes of the richest purple felt. And there was one cloak which was long in the weaving for him, a magnificent work, on which was represented the world and the heavenly bodies; this was left behind half-finished when the reversal of his fortunes came, and no succeeding king of Macedonia ventured to use it, although not a few of them were given to pomp and luxury.
(The Loeb Classical Library, translated by Bernadotte Perrin. Electronic source: Plutarch’s Lives on William P. Thayer’s Web Site LacusCurtius.)