Open the attached form in Qt designer (4.5.1/4.5.2 up to 4.6.1, Windows only). The form contains 4 comboboxes, which are enabled or disabled and editable or not.

When previewing the form, you'll notice the background color is different for disabled comboboxes given their editable state. For enabled comboboxes, there is no visible clue to distinguish between the editable one and the other. Background color of a combobox shouldn't depend on its editable state.