Alas, a PopupMenu is no Component. It uses a MenuWindow as Component instead. But that is not easily accessible (if at all). Hence there is no setColour() on PopupMenu.
I prepared a PR to store a SafePointer to the targetComponent and use it in findComponent of the PopupMenu::LookAndFeelMethods.
If somebody had answered in the feature request thread, I could add it there, but the forum doesn’t allow monologues 