Most of the time black and white appear as opposites. White means light and purity. Black is darkness and evil. But not always. Sometimes the words are used differently.
White hats and black hats represent the traditional meaning of the words. The hats are symbols of the "good guys" and the "bad guys" in American western movies.
White hats and black hats had a real purpose in the early days of motion pictures. The early movies were made with no sound. So, the movie directors put a white hat on the hero and a black hat on his opponent, the bad guy in the story. That way even a small child knew immediately who was the hero.
The tradition of white hats and black hats is no longer a part of movie making. But you still hear the expression when people talk about good guys and bad guys.
A "black sheep" is a person who does things that are not accepted, that violate tradition. A black sheep is rejected because he brings shame to his group. A family may have a member who is thought of as a black sheep, a person who is not welcome at family gatherings.
Black is also used in some expressions that describe good things. Being "in the black," for example, is a good situation for anyone. It is a business expression that means a company is earning money. A store mat makes a profit is "in the black." The expression comes from the color of ink that is used in the books that record a company's profits or losses. Profits are written in black ink. Losses are written in red. So, when someone says his business is "in the red," he means it is losing money.
White usually means something good. A "white-collar" job, for example, is the kind of job many people seek. It is a job where you work at a desk, using your brain instead of your muscles. Usually white-collar jobs pay more than "blue-collar" jobs. Those are jobs where you use your muscles.
Sometimes white is used in an expression that is not good. Whitewash is such an expression. At first, whitewash meant to paint over something with a white paint to make it look better. But now "whitewash" has a different meaning: to hide or cover up mistakes or failures.
A newspaper might report, for example, that the investigation of a train accident is a whitewash, because investigators did not really try to discover who was responsible.
Some expressions use black and white together. Someone may say to you that your proposal sounds all right, but he wants to see it in black and white. He wants to see your proposal in writing-black ink on white paper.
Some people are said to see things only in black and white. In this expression, black and white represent opposite extremes, with no middle ground between them. Most issues in the world are not so simple as black and white. One side of an issue is not all white-right or good-and the other side is not all black-wrong or evil. In the real world there are many grey areas where black and white mix to represent the true situation.