Usage/connotation of "いろ" meaning someone's "girl/lover"

Question

In Rurouni Kenshin, a woman is running away from Kanryuu's men. A guy asks her "are you Kanryuu's girl?" (anta wa Kanryuu no iro ka?)

Anyways, my question is the usage of "iro" to mean one's girl/lover (since I haven't heard it before). Is it an out-dated term (Rurouni Kenshin takes place during the early Meiji era iirc)? Is it considered vulgar/impolite? And is it usually written in kanji, or hiragana, or what?

Answer 1

It is listed in goo dictionary as follows:

８

㋐情事。色事。「―を好む」「―に溺れる」

㋑女性の美しい容貌。「―に迷う」

㋒情人。恋人。いい人。「―をつくる」

It is simply dated and vulgar (impolite does not seem to fit - it's not really used now). My impression is that it is often used in Jidai-geki (=Japanese historical drama mostly set in Edo era, which was popular in Showa Era).