You 油 probably refers to youjuan 油絹, a kind of fine or oil-coated silk. Mang 蟒 [python] is a commonly seen pattern in male clothing, often embroidered, indicating their high status. Python robe can be worn under the armor. See Liu Yonghua 劉永華, Zhongguo gudai junrong fushi 中國古代軍戎服飾 [Ancient Chinese Armour] (Beijing: Qinghua daxue chubanshe, 2013), p. 253, picture 16-15.
