noun, noun which may take the genitive case particle 'no'
rainbow-colored; iridescent
Describes something that has the colors of a rainbow, often used for visual descriptions.
虹色のシャボン玉が空に浮かんだ。
Rainbow-colored soap bubbles floated in the sky.
彼女は虹色に光るドレスを着ていた。
She wore a dress that shone in rainbow colors.
虹 is the noun 'rainbow', while 虹色 is the adjective/noun meaning 'rainbow-colored'.
Compound of 虹 (niji, 'rainbow') + 色 (iro, 'color').