You have to let them go through the “training” part of the potty training (either by someone else or on their own) before the can be coaxed into using it. You can see the training process in the queue of the adult that’s training them or their own if there’s no adult. As far as I know it doesn’t matter if they go into the red once they're on the potty and have started the animation for it.
If nice they reach Level 3 I believe they’ll go right away. I haven’t really paid attention to it I guess lol.