In practice what you wear often depends on the coach or club. In some clubs and schools you can wear whatever shirts you want, but in others there are certain colors or practice shirts that they want you wearing to practices. For Kennedy High School, we get gray practice shirts that we have to wear every practice and we have to wear them with black shorts unless they say otherwise. At our club, we have practice shirts that we wear with black or navy blue spandex. There are two different colors of practice shirts and each one is designated to one practice night, so the black one is for Tuesday night and the gray one is for Wednesday. Some coaches are really strict with the uniform rule, like our club director who makes you run for wearing the wrong thing, or they can be more relaxed about it and not really mind when you wear the wrong thing as long as it only happens once in a while.
There are also some optional things that some people wear, like bows. Some teams will get matching bows to put in their hair for tournaments, which I personally don't use. Teams can also decide to match things, like kneepads and socks, by all wearing the same color.
There are also some things that you are not allowed to wear when you play a match. You can not wear jewelry, bobby pins, and any other accessories that might cause a problem or fall of when you play.