I just heard this colloquial sentence on YouTube:
Mais je suis sûr que je peux lui rendre le sourire si je l’emmène manger un parfait arc-en-ciel, t'en fais pas !
You cannot eat a "rainbow" to begin with. The possibility of "poisson arc-en-ciel" occurred to me, too, but there is no particular mention of any fish, either.
Then I wonder... Could it be about eating various fruits and vegetables of different colours as diverse as those of a rainbow?