Forum:Uniform

I have a question I'm hoping someone can help me with. I am wondering about the first years' uniforms for Hogwarts. They purchase and wear their uniform before they know what house they're going to be in, so how is it that later we see them wearing uniforms clearly customized for each house or at least indicia, such as a colored hood that shiows what house they are in? And also in the first film I noticed their uniforms were plain at the sorting, yet somehow later developed each house's corresponding colors?