The one time I don't bring a real camera, this beautiful full rainbow appears, and at the end of the rainbow, a bookstore. Each book a treasure, and a pot of gold at the end of the rainbow. Sure, a smartphone can record it, but a smartphone just doesn't have the dynamic range or color depth needed for challenging lighting conditions, or the subtlety of a rainbow's colors. There simply aren't enough photons hitting that tiny camera sensor.
Not as vibrant as it could have been. But it'll have to do.