How did the colonists dress at the Boston Tea Party?

While this protest was illegal, making any old method of obscuring individual identities necessary, Mohawk warriors were chosen because they were a truly American symbol of freedom and rugged individualism. This is a parallel to why the Americans also began to identify with the rattlesnake, bald eagle, and turkey (which Thomas Jefferson wanted to be the national symbol). Mohawks, bald eagles, and the rest were things unique to the New World.
Essentially, the Sons of Liberty (who conducted the Stamp Act protests) were showing that they identified more with American symbols than English ones.