Supported formats are #RRGGBB and #AARRGGBB. Hill shading color. The alpha component is combined with the "magnitude" attribute - you can use both to control the opacity of the hill shading. They are multiplied, so for example a magnitude of 128 and a color of #ff000000 would result in an opacity of 50%, while a magnitude of 128 and a color of #80000000 would result in an opacity of 25% (0.5 * 0.5 = 0.25).