Those stupid rainbow monkey things from that show on Cartoon network (Yes I did just only remember those cause it was in Foxxies story >____>)
They are symbolic of snakes.... Poisonous, Deadly, frightening creatures, and are therefore all those things themselves...
They give people false hope, which is then exploited by the malicious snakes that run the world in order to tear them down.
They're not solid so any poor idiot who thinks they can climb a rainbow will be really ticked when they can't seem to touch the rainbow while the rainbow is laughing evily at it's deceptive powers.