Yes, unless the colour is specified, they're black. And they're only knee length - you aren't important enough to get a full length gown until graduation.
And they are also open at the front, so no, you don't just wear underwear! Although that could be an interesting alternative... Boys mostly wear suits, although sometimes it's more relaxed and they wear smart casual under it. Girls wear smart stuff - I usually wear a black skirt and some kind of nice top.