Symbolically, rainbows often represent a bridge between heaven and earth. They’re not unlike angels, who also act as mediums between the dimensions.
In both the RWS and Thoth decks rainbows are a featured symbol in card #14, though in the RWS version they’re more hidden than in Thoth.
In the RWS deck, the Temperance card pictures an Angel pouring water between cups at a magical angle. On the angel’s chest is the symbol for fire.
The bridge between dimensions is of course reflected in the angel herself, but there are also two irises in the picture.
The word rainbow comes from the Greek word iris, which was the name of the goddess credited with the creation of rainbows. She was also known as a messenger of the gods.
In the Thoth deck, the Art card shows a two-headed, multi-breasted Artemis mixing fire and water over a cauldron. Her gown’s lapels are rainbows that seem to grow from the mist of her concoction.
Both cards suggest a balance between the opposing energies of fire and water. Such a mix can create something as extraordinary as a rainbow, or a bridge to another world.
The video below shows a very unlikely example of this same idea.
In it a woman named Kristi mixes the firepower of her shotgun with the water of a running stream. And just like Iris and Artemis, she manages to make herself a rainbow.