A senior serviceman wears a black skirt, an ordinary serviceman wears a yellow skirt, and a junior serviceman wears a skirt of mixed colours, that is black at the front en yellow at the back.
(Office titles taken from Charles O. Hucker, A Dictionary of Official Titles in Imperial China)
But in the Shijing it is linked to the primary wife (as opposed to a concubine):
Green is the upper robe,
Green the upper, and yellow the lower garment!
The sorrow of my heart, –
How can it be forgotten?
It is said that this poem tells about a first wife (the lower yellow skirt) who’s position is being threatened by a concubine (the upper green robe). The first wife should wear clothes of the same colour, and mixing colours was a sign of inferiority (just as with the xiashi 下士, the junior serviceman). See the commentary in 毛詩正義 (search for「綠衣黃裳」).
Deng Qiubai 鄧球柏 says in his commentary to the Mawangdui text:
‘Yellow skirt’ means ‘yellow clothes’. These clothes were worn during the year-end sacrifice. In the Liji it is said: “yellow clothes and a yellow cap for the sacrifice”. Some scholars write 裳 (instead of 衣).
Deng Qiubai 鄧球柏, 《帛書周易校釋》, p. 265
Whatever it refers to (an ordinary serviceman, a first wife being forced into inferiority or someone performing a sacrifice), it tells about a humble position.
The Mawangdui text has chang 常 instead of chang 裳. Both characters share the same origin and are often used as each others loans. Wang Li says about this:
當下衣講，實為一詞的不同寫法。(…) 常又訓旌旗，是常（裳）的引申義。(…) “裳” “常”分用後，“裳”不用於旌旗義。但與“常”為同源詞。
When we talk about “常” and “裳” as related to clothing they are in reality one word in different styles of writing. 常 is also explained as ‘banners and flags’, it is an extended meaning of 常(裳). Later on “裳” and “常” are differentiated, however “裳” is never used with the meaning of ‘banners and flags’.
Wang Li, 《王力古漢語字典》, p. 1224
It is tempting to read the Mawangdui text as ‘yellow banner’ (as I did years ago), but since the received text speaks of 裳, which was never used with the meaning of ‘banner’, and since the phrase 黃裳 occurs in many other texts as well, I think we should just translate it as ‘yellow skirt’.
Yellow skirt. Greatly auspicious.