In Korean tradition, dreaming of a rainbow is read as very auspicious. It is most often tied to 전반, 애정.
A rainbow is a classic sign of good fortune after difficulty — gazing at one for a while, or passing beneath it, suggests a wish coming true.
Tradition reads the symbol as an omen; psychology reads the same dream as a mirror of your present state. Describe your dream and both readings appear together. Interpret your dream