A colon usually indicates a pause after a cue or suggestion, or indicates a hint for the following context or summarizes the anterior context.
A colon is used in the following cases:
Used after the general or suggestive words to prompt for the following context 用于总说性或提示性词语后
北京紫禁城有四座城门：武门、神武门、东华门、西华门。The Forbidden City in Beijing has four gates: Wu gate, north gate, Donghua gate, Xihua gate.
她高兴地说：“我们去好好庆祝一下吧”。She said happily, “let’s go and celebrate”.
Used to summarize the above-mentioned points 用于总结上文
张华考上了北京大学；李萍进了中等技术学校；我在百货公司当售货员：我们都有光明的前途。Zhang Hua admitted to Peking University; Li Pingjin went to a secondary technical school; I worked as a shop assistant in a department store: we all have a bright future.
Used after the words that need to be explained, introducing further details o explanations 用于需要解释的词语后边，表示引出解释或说明。
After the address forms of a letter or speech. 用在书信、讲话稿中称谓语或称呼语之后。
同志们，朋友们：现在开会了……Comrades, friends: We are meeting now.
A colon can not be used in a sentence in which there is already one. If the second colon is necessary, a new sentence must be made.