Realistically I think the underwater.
hull would still be red. I don't think they had time to get her in a drydock to paint the whole hull.
For the movie several subs were used!
The pink one was a Balao class and it was full pink: there are some underwater sequences showing the pink hull ( also if I agree with your logical comment).
BTW I guess that Us ww2 subs were having grey hulls.