Police Uniforms Act Article 4

The police may wear the working uniform while performing routine duties and attending assemblies unless the standard uniform is considered more appropriate. For the needs of a mission, , however, the police are not required to wear the uniform.When wearing the uniform in accordance with the requirements in the preceding paragraph, the police may add an overcoat or raincoat if it’s cold or rainy.
中文原文 警察服制條例第4條
警察人員平日執行職務及參加集會時,除以服用常服為適當者外,均得服用便服。
但因任務需要,得不服用制服。
依前項規定服用制服時,因氣候寒冷或下雨,得加服大衣或雨衣。

English translation for reference only; the Chinese text prevails. Not legal advice.