In this world, there are not only various kinds of animals, but also a variety of kind of human. Therefore one should never judge a person by external appearances, but should judge by his real behavior.
Nowadays, some people do not do things that they really want to do. For example in the office, someone like flattering his boss for his good promotion. It means that he usually acts as he is working hard when his boss is in the office, but whenever his boss is not be there, he will not work at all. If new employee is just received to work there, he may be thing that flatter guy is the good person who enjoy working, but next few days, he will know that actually flatter guy is only the lazy man who usually act like good worker.
Another poit of view, there are many people who are not candid. They usually have secrets, and lie. Although they talk and smile to you, in fact they want to hurt you. They may be gossip you with your colleague or your boss, so you cannot judge a person without knowing him.
In addition, sometimes, you may thing that people who are good looking, or rich will be the good one. Someone look so handsome, but his behavior is really bad such as he enjoy hurting woman, or does not take care of his parents at all. On the contrary, someone who are not handsome and sometimes looks greedy, he may be polite guy. Therefore you should not judge a person by only external appearances.
To conclude, there are various kinds of people such as black, white, handsome, tall, and so on, but all of these are external appearances which can not be the gain to judge people. Someone may be frank, and tell the true thing, but someone may usually lie. Therefore one should never judge a person by only external appearances, but should judge by his behavior after really knowing him.