Depends om the mood the shot is trying to convey. Just cause it would look pretty doesnt mean it’s right for the scene. Also rainbows tend to go away and you dont want a rainbow in some shots and not in others. It’s a shot for a movie and rainbows are distracting and tend to be associated with light heartedness and or hope. If you dont want your scene to convey that, its generally good to not have a visual reflection of it