Dual nationality, not sure how to switch passports! [duplicate]

The only significant restriction I know of is that you must enter the US with your US passport (there may be a restriction by the country of issue of your EU passport, but probably not). Therefore you need to carry both passports.

It doesn't matter which passport you book your outbound flight with, but it's probably a good idea to book your flight back to the US on your US passport, which if it's a return flight implies booking both on that. If you are only transiting in Paris you shouldn't need to pass border control, so it shouldn't matter which passport you use there.

You will need visas for many African countries, so present the passport that you used to get the visa when you enter that country. Present the passport that you used to book a flight when you get on that plane. Present the passport that you used to enter a country when you exit it. Should be a simple enough as long as you don't confuse airline checkin with outgoing border control.

Since you are a US citizen, you don't need to worry about the US using passenger records to implement the electronic I-94 system, and the possible confusion that using multiple passports may cause there.

You will obviously require your US passport upon returning to the US. You can book all your flights using your EU passport, and keep your US passport somewhere safe for the duration of your travels.

On your final flight back to the US, you may be asked about the lack of an ESTA on your EU passport to enter the US (which normally an EU passport holder would require). However, showing your US passport to the airline agent at that time should clear up any question about whether you will be allowed to enter the US.

You generally must always enter and exit a country on the same passport.

The passport you exit a country with does not have to be the same one as the one you enter the next country with.

I have two passports, I generally book my flight using whichever one works better at the destination. I leave my country of residence using my passport-with-visa. There appears to be no connection between immigration control and the airlines - the airlines want to know that you are able to enter the destination country (or they get a nice big penalty) and my country of residence simply wants to know that I didn't overstay my visa.

On more than one occasion I have used the "wrong" passport to book the flight and simply handed the other one to the destination's immigration. They don't even blink.