Three health codes were tied to a mobile phone, and suddenly one turned yellow. Why?

Three health codes are tied to a mobile phone, and suddenly one turns yellow because the original user's itinerary of this health code is the same as other red codes. According to relevant data, the yellow health code is caused by the overlap of travel codes and users' travel with red team. Health codes are based on real data and can be generated by citizens or people who have returned to work through online declaration and background audit.