In the 16th and 17th Centuries, Puritans believed in "purifying" the Church of England from its Roman Catholic ancestry. Puritans had strict beliefs about the way people should behave, and their dress reflected those beliefs.
In the early colonial period of United States history, the Puritans believed that their religion was the only path to God's salvation. They were also the people who brought the first public schools to America, because they believed that every person should be literate enough to read the Bible. Puritans believed in simplicity and avoided excess, which was reflected in the way they dressed.
The myth is that Puritans always dressed in somber black, with a complete lack of ornamentation. In reality, black dye was very expensive in the 16th and 17th Centuries and would have only have been worn for formal occasions or during church. It is also a myth that Puritans would not wear clothing that was brightly colored or embellished with lace and other trimmings.
Dressing For Their Station
Puritans believed that acting above their station was the same as lying or trying to fool others into believing they were something they weren't. So, working class Puritans would have dressed plainly and in drab colors, because that was what they could afford and what others of their station wore. Wealthier Puritans or those of higher ranking could afford brightly-colored cloth and lace or other trimmings, so they wore clothing made from these.
Puritan clergy were university educated and dressed to reflect their station in life. They wore black to show that they were of a high social rank. Black fabric was the most expensive at the time, so being able to dress in it all the time meant that the individual had status and money.