So, if I understand correctly, the colors for combo boxes were set in the actual combo boxes without the LAF
I just reread your exampe and noticed you were using the popup menu LAF functions. The popup menu is not part of the combobox component, so there are no colours in the obj parameter.
A good practice for anything regarding LAF customization is to always use Console.print(trace(obj)) as first line of the function, then you'll see everything you can work with.