You may or may not already know this. Here’s how Williams adopted the color purple according to the report by Eugene M. Jerome, Class of 1867.
Early in the summer of 1865 the Williams baseball team were about to leave Williamstown to play the final game of the series against Harvard, and just before our departure two young ladies–my sister and my cousin*–who were very much interested in the result of the game, learned that while Harvard had adopted magenta as its College color, Williams was without any. They hurriedly purchased some royal purple ribbon and made small rosettes out of it, and pinning one on each member of the team, said “Let this royal purple be the Williams color, and may it bring you the victory over Harvard.”
“The royal purple joyfully floated from the mast-head that next day, for the game was handsomely won. This victory gave Williams the championship in the intercollegiate contests between Princeton, Harvard and Williams, and the royal purple has ever since been the banner under which Williams has won so many brilliant victories.
*Jerome’s cousin was Jennie Jerome who would became Lady Randolph Spencer Churchill, mother of Winston Churchill.