As an attorney, I have seen lots of witnesses in court. I have seen everything from t-shirts and ripped jeans to (I am not kidding) a prom dress. Basically, think about the impression you want to make because first impressions matter in court. I don't think you need a suit or anything but a nice pair of pants, blouse or nice top with maybe a cardigan (it can be cold in court rooms) or a casual dress (no cleavage exposed, etc.). Basically error on the busines casual side. As for shoes, I don't think open-toed are a no-no if they are a little more conservative -- no wild shoes. I would probably try to a pump (even with open toe) or low kitten heel slides. To be honest, the shoe is really the least important part in this scenario. If you actually get to testify (which by the way, court never happens when they say and cases get continued constantly), the only time anyone will see your shoes is when you walk to and from the witness stand. Most witness stands that I have seen our enclosed so your feet and lower body are covered.
