I'm sorry to hear about your friend's mother. It's thoughtful of you to be attending. I actually went to a funeral yesterday and saw very little variety - most people wore black with the occasional navy. The light colors were mostly men's shirts under their suits. I wore a black wrap dress with a black shirt underneath for coverage, but I'm sure a light or dark neutral would have worked, as well. I also chose to wore nylons (hose) because I was not sure about exposing my legs. This ended up being a great choice because it kept me warmer. For me, erring on the side of covered up and dark made me feel comfortable and respectful. Joy's point about shoes is very good if you are going to the burial.
Also, keep in mind any particular cultural traditions there might be (there may not be any in this instance.) I was one of three Caucasians at an all-Chinese funeral so I made sure to ask my friend beforehand if there was anything I should be aware of. If you don't feel comfortable asking or if your friend is quite distraught and you do not want to bother her, look around online to see if there is any information.