I’m not sure I understand this correctly, but here is how I think it works:
When the game starts, Belgium is an ally that has been conquered. No one actually plays Belgium and the Dutch have no government, no capitol and no military units, but the former Dutch colonies are still allies. So an allied air unit can land there just like a US plane can land in Morocco after Paris falls.
The difference between Belgium and France after Paris falls is that France has military units that are played by the France player as their goverment (in exile?). Pro-allies neutrals like Greece have their own governments intact and all they will choose to join the Allies if attacked or activated.
Former Dutch colonies are allies but they have no government so they are up for grabs (Anarchy!). Whoever can get a land unit (not AA) there to establish a local government takes control. In the meantime the allies can land air units there but do not take control of them.
If a former French colony is taken by an axis power and then taken back by an ally of France, then it is provisionally governed by that ally until Paris is liberated.