The clothes of the Hong Kong police.

It is right for the police to wear work clothes in downtown Hong Kong, and it is also right for other ranks of police officers and police chiefs to wear blue shirts. The policemen in yellow-green uniforms you saw when you passed through the customs are policemen serving in mobile units (also known as PTU and police mobile units, mainly to prevent riots). They were sent to the customs to help the local police, but you happened to see them. In fact, sometimes there are opportunities to see policemen wearing brown-green camouflage patterns at the pass. They are policemen with special tasks.