What do the three-level travel warnings in Hong Kong stand for: yellow, red and black? Why is there such an outbound warning?

According to the early warning system for outbound travel in Hong Kong, which came into effect on 10, 2009, the early warning system for outbound travel in Hong Kong is divided into three levels: yellow, red and black, with yellow as the lowest level. When there is a threat in a certain area that may affect the personal safety of Hong Kong tourists, the SAR Government will issue a yellow warning to remind the public to pay attention to the local situation and be vigilant; A red alert means that there is an obvious threat in a certain area. Hong Kong tourists should adjust their itinerary and avoid going to relevant areas if it is not necessary. When there is a serious threat in a certain area, the SAR Government will issue a black warning, calling on Hong Kong people to cancel all travel itineraries and not to go to the relevant areas.