A rainbow-coloured dragon is quite uncommon.
The exception is the Rainbow Serpent, or Rainbow Snake, found in traditions across the world.
The Rainbow Serpent devours people and vomits them back up, symbolising death and rebirth.
Some dragons are associated with the rainbow in other ways.
For example, the Aztecs said that the dragon god Quetzalcoatl could be seen flying across the sky in the form of a rainbow.
Rainbow dragons are inherently connected to the weather, specifically the rain, since the natural manifestation of the rainbow is after a rainstorm.
Dragons of Fame: Quetzalcoatl, Aido Hwedo / Dan Ayido Hwedo, The Rainbow Serpent
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Source - http://www.blackdrago.com/colors_rainbow.htm
Image - yuuza.deviantart.com
