I disagree with the scarlet balloon. No one calls a reddish balloon a scarlet balloon. It's basically a compound English word to stand in for any colorful balloon. While you can have a green balloon, "green balloon" does not invoke joy and fun. There was a popular song in the 80's that might have influenced or established the connection.
If the title would have been "the red balloon", I would agree, that you could change the color to something more memorable.
Curiously enough, I would accept joy balloon as a synonym for red balloon already.