They should also obey the colours set to the LookAndFeel. In the juce LookAndFeels the PopupMenu of a ComboBox only follows the LookAndFeel, not the ComboBox it is targeting, see here:
You can find fixes in the various posts in that thread where I worked around it, but they all involve patching the LookAndFeel to get the colour from the targetComponent rather than from the LookAndFeel.findColour().