John Gill Commentary Mark 15:17

"And they clothe him with purple, and platting a crown of thorns, they put it on him;" — Mark 15:17 (ASV)

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), (See Gill on Matthew 26:29).