I agree with mepal regarding shells under gowns. If I did saw someone wearing a sleeveless gown with a contrasting shell underneath, I’d probably think that she preferred not to invest the time/ money to have it altered, or actually preferred to wear it that way (though I’d prefer not to wear that myself because I don’t like the way it looks). And if I thought anything about the woman’s tznius, it would probably be, “it’s a kiddush haShem to see women who are eager to fulfill the mitzvah of tznius, even if it means they are perceived as compromising fashion”. Not to go all Reb Levi Yitzchok m’Berditchev on you.
NY Mom, I do agree that teenaged girls need to be careful of they way they dress both for tznius reasons and safety reasons.