Duke Ellington and his orchestra recorded a famous song Mood Indigo about the deep blue color,indigo.

VOA: special.2009.11.15

They seem to sort of reflect deep differences in your mood and emotions and thoughts.

他们反应出深层次的不同,在你的情绪，情感及思想中。

耶鲁公开课 - 心理学导论课程节选