Can you have viral infection with yellow mucus ? or when it turns yellow is it bacterial and time to go to doctor?

As a rough rule: of thumb, if you have had a throaty cough or runny nose which started clear for more than 3-4 days, and the color of the mucus changes to yellow or green and consistency to thick, then there has been a change in the condition. Most of the time, it means there is possibly some bacterial suprainfection occurring in those areas. It is likely best to be seen by a healthcare provider for exam / Dx.