Before Ching-Ying Lam, no one had made a movie about zombies, and in Ching-Ying Lam's movie, zombies appeared in front of the audience for the first time in the official clothes of the Qing Dynasty. At that time, Ching-Ying Lam played a Taoist in Mr. Zombie, and it was from then on that zombie costumes began to take shape.
As Yu said, zombies are hierarchical. If I remember correctly, it was white first, then green, then purple, and I forgot a few grades later. Judging from the speed of their evolution, when they evolved into the zombies we saw in the movies, it happened to be the Qing Dynasty, so the zombies all wore the official uniforms of the Qing Dynasty.
Most of the official uniforms in the Qing Dynasty were the same. Regardless of the size of the official position, everyone's official clothes look similar. As we all know, the Qing Dynasty itself was a relatively corrupt dynasty, so its official clothes also gave people a very gloomy feeling. Just suitable for the gloomy temperament of zombies, so zombies in Hong Kong ghost films are dressed like this.