Rainbows have been a popular symbol in many cultures for centuries and they have a variety of different meanings. In the world of Pokémon, rainbows often appear on cards associated with the Legendary Pokémon Ho-Oh. Ho-Oh is said to represent the hope and happiness that come after a storm, which is why rainbows are often seen as a positive omen.
In some cases, however, rainbows can also be indicative of trouble. For example, in the anime series Pokémon: Advanced Challenge, there was an episode where Ash and his friends came across a rainbow bridge that led them to an alternate dimension. This other dimension was filled with dark and dangerous creatures, so the group had to be very careful when crossing the rainbow bridge back into their own world.
Overall, rainbows are generally seen as positive symbols in the world of Pokémon. They represent hope, happiness, and new beginnings; perfect sentiments for any young Trainer just starting out on their journey!