虹 is one of the most commonly used Japanese kanji, and it also has a lot of special readings in addition to the standard onyomi and kunyomi readings.
ONYOMI 音読み
にじ niji
KUNYOMI 訓読み
コウ kou
Examples
虹は自然現象である。A rainbow is a natural phenomenon.虹は7色だ。The rainbow has seven colors.何と美しい虹だろう。What a beautiful rainbow!虹は最も美しい自然現象の1つだ。A rainbow is one of the most beautiful phenomena of nature.窓の外を見ると、虹が見えた。Looking out of the window, I saw a rainbow.雨が止んだとたんにきれいな虹が出た。No sooner had it stopped raining than a beautiful rainbow appeared.虹は空に弓じょうにかかる。The rainbow forms an arc in the sky.虹の中にいくつの色が見えますか。How many colors do you see in the rainbow?空に美しい虹がかかっている。A beautiful rainbow is spanning the sky.雨がやんだとたんに虹が現れた。Scarcely had the rain stopped before a rainbow appeared.