Combo Box Backcolor problem

I change the colors of input boxes to slategray when I want to disable user
input. Works fine for a combobox with the "dropdownlist" type selected.

However, when I use a combobox with the "dropdown" type selected and I set
the backcolor to slategray, only a thin line around the box resets to
slategray, the rest of the box sets to the default Windows gray.

Am I doing something wrong?

Thanks.
--
Mark Lancaster