And they clothed him with purple, and platted a crown of thorns, and put it about his [head],
Note 2 at Mr 15:17: Mark and John recorded this robe as being purple, while Matthew said it was scarlet. Depending on the shade of purple or scarlet, these colors are close enough that this could not be construed as an error on Matthew's part.
It is probable that this robe was one of the Roman soldiers' capes that adorned their uniforms.
Note 3 at Mr 15:17: This crown of thorns mocked the wreath that Caesar wore.