A somewhat literary or old-fashioned term for a love letter; in modern casual speech, ラブレター is more common.
She keeps an old love letter carefully in her desk drawer.
Writing a love letter is a rare custom nowadays.
Compound of 色 (iro, 'color; love; passion') and 文 (fumi, 'letter'). The exact historical derivation is uncertain, but the kanji 色 has long been associated with romantic or erotic connotations.