Why do books have different color spine labels?

Chinese — CH
English — EN (White label)
Japanese — JA
Spanish — SP

For quick visual language information, the spine label of each book is color-coded to indicate the language of the text. Bilingual books are given call numbers reflecting the track language but have small color-coded dots on the spines to indicate other languages inside. For example, a book with a blue
spine label and a white dot indicates a book with text in Japanese and English.