In the late 1800s, universities in the United States adopted the standardization of cap and gowns students wear to graduation ceremonies:
European institutions have always had diversity in their academic dress, but American institutions employ a definite system of dress thanks to Gardner Cotrell Leonard from Albany, New York. After designing gowns for his 1887 class at Williams College, he took an interest in the subject and published an article on academic dress in 1893. Soon after he was asked to work with an Intercollegiate Commission to form a system of academic apparel.
The system Gardner Cotrell Leonard helped form was based on gown cut, style and fabric; as well as designated colors to represent fields of study.
COLOURlovers has the breakdown of color and field of studies (music is pink!): Link