“One’s appearance must appear refined and respectable.”
See Mesilas Yesharim 23 in the opening paragraph (“Path of the Just” page 163, Feldheim) where the Ramchal explains that tznius clothing are those which are respectable (??????????) but not “overly fancy”. Although Mesilas Yesharim is a “mussar” text which guides people to do more than the minimal requirement of halachah. Nevertheless here the Ramchal is defining tznius clothing, which should be worn to develop humility, rather than giving a “higher level” of tznius.
Bright colored clothing or clothing that otherwise attracts attention are not permitted.
See Rama Y.D. 178:1, Kitzur Shulchon Oruch 3:2, note the Shach 178:3 who explains that black is a modest color.
A married woman must cover her hair.
See Chayei Adam 4:5 which cites Bamidbar 5:18.
An unmarried woman’s hair should be short or tied back, such as in a braid.
This reflects the notion that one should look respectable but not over made up. Torah sources indicated braided hair (see Shabbos 95a) and hair pulled back (see Misdrash Shir Hashirim 4:3) is attractive, but not ostentatious like those described in Yeshayahu 3:16 (see Rashi there).
Makeup and perfume is intended to beautify a wife for her husband, beyond this it should be used scarcely if at all.
See Tanchuma Vayishlach Piskah 5