Teen, there ARE many things that people do that are a matter of custom and taste, and many things that are halacha l’maiseh. I don’t want to get into a whole tznius discussion here, so I will just say that some things that SOME people consider to be not tzanua, may be considered so by others. Some people will only wear the colors black, brown, navy, and white. They consider green, yellow,purple, pastels,etc. not to be tzniusdig. Almost universally (and I said ALMOST) red is considered an immodest color because it is very attention-getting in a potentially negative sense (which is a core reason for hilchos tznius). The point is, within your own kehillah, there are standards by which the klal lives. Another kehillah might have a different opinion, and in some cases it may simply be a matter of personal preference, or it may be a halachic perspective. I honestly do not get the whole issue with cap sleeves and long sleeves underneath, but al taam v’reyach ein l’hitvakayach.
