I've tried changing it in Unit -> Model -> Color with no avail. Does it [not] work for anyone else?
you can practically modify every value that you can see when you convert it into raw data, by selecting the "view raw data" field. (I'm not saying that it's not at least wierd that you can't change it ^^)You see how the data field has four fields, and disregarding which color you choose, the first one is always 255?
My take is that this first value controls the transparency, and since it's always set to 255, we never see any color change. Perhaps Blizzard forgot to add a slider to control it?