James,
Does it look right at design time? When I do the two steps below I see black text on a yellow background at design time and runtime. Can you test this on another machine?
>I put a base class combo on a form set the back color to 255,255,128 and set the style to 2 - dropdownlist. The text is black with a grey background only around the text. The rest of the backcolor is yellow. The grey is only seen if there there is text showing. I went in the init and added a 4 lines with additem. If you don't see this then maybe is is a video card problem?