Legal analysis: Generally speaking, writing on formal occasions should use a blue-black pen or a black carbon pen. This is because words written in blue-black ink and black carbon ink are not easy to fade in water and oil. There are also some inks that are not suitable for writing, such as red, or too light in color, such as green and yellow, or difficult to preserve, such as ballpoint pens (whose oil core melts easily in oil).

Legal basis: Article 490 of the Civil Code of People's Republic of China (PRC). If the parties enter into a contract in the form of a contract, the contract shall be established when the parties sign, seal or press their fingerprints. Before signing, sealing or fingerprinting, one party has fulfilled its main obligations, and the contract is established when the other party accepts it. A contract shall be concluded in written form as stipulated by laws, administrative regulations or agreed by the parties. If the parties do not do this in writing, but one party has fulfilled its main obligations and the other party accepts it, the contract is established.