In the Save For Web window, when you select a transparency matte color, there's no way to select an exact color without typing in the numbers manually. i.e. you can't paste a hex code in the hex field like you can in other places, and you can't use the eyedropper tool to select a color in the toolbox or image window (if you can even see them to begin with, the Save For Web window is so frickin' huge).