I have the same issue and I think colour distinction isn’t enough to distinguish. In addition, it should not matter which tab you are on it should be clear at all times which account you’re using.
Sometimes you log into the joint account to check on things and then you close the app. The next time you go into the app maybe to just quickly pay a friend or something you will probably not think twice and accidentally pay out of the joint account as it was the last account you looked at. Least that what my one does.
It’s happen on more than one occasion not just for me but also for the person I’m sharing the account with.
I mentioned in my post on this topic that a banner would suit quite well to solve this.