Phlegm color changes based upon the death of white blood cells and their type. Yes, bacterial infections tend to result in a yellow phlegm and even a greenish hie when seriously ill. The cells putrify so there is a distinct odor.

Your mucus should be clear and if it changes while you have influenza, you likely have an increase in temperature (higher fever) and a secndary bacterial infection, aches/pains, chills/shivering, etc.

Physicians listen for these descriptors as does the nurse when taking a patient history.