The taskbar is black because the image in the Aero theme is black (or very dark). It is most likely dark because it has pre-multiplied alpha.
If you want a taskbar of different color then you will have to edit the theme file. BTW, if you are going to be editing the bitmap in the theme you might as well make it opaque and then you won't need the Classic Shell setting.
Keep in mind few things:
- the taskbar is not solid color. It has a border and a slight gradient.
- there are 4 images in the theme file, for top, bottom, left and right taskbars.