Can I travel within the EU with a WA residency card?

Both your passport and residency card will be checked if you enter or leave the Schengen area

Can I travel within the EU with a WA residency card? I HAVE a Withdrawal Agreement (WA) residency card – can I travel throughout the EU using this card or do I still need to carry my British passport with me? 

As a foreign resident of France with a residency card, you can travel freely in other Schengen area countries for up to 90 days in any 180-day period but paperwork is still needed. You should always, in theory, have your residency card with you in France in case of a police check, and this card can also be used as an identity document for internal flights in France. 

You should take both with you on trips outside France within the Schengen area, though there will be no automatic border checks if you are not travelling by plane. On flights you will be asked to present your passport as proof of identity. Also, both your passport and residency card will be checked if you enter or leave the Schengen area – for example, if going from France to the UK.