system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does snot go yellow when dealing with an infection and green when it gets worse?
gpt
Color doesn’t really mean a lot.  Even red isn’t that interesting, unless your sneezing out whole blood clots or something. 

What does matter is symptoms associated, time period, and “thickness.”  

If you had a fever two days ago and you tell me you had green, yellow, or even red streaked snot today, that’s meh.  (Viral sinusitis)

If you’ve had thick yellow mucous for 2 weeks after a upper respiratory tract infection without shortness of breath or chest pain with inspiration, that’s pretty meh as well.  (Probably Bronchitis)


One thing that is of particular interest is something called double sickening.   This is the person who has a upper respiratory tract infection (cough, sore throat, fever, runny nose, etc) for a few days, reports getting better, than starts developing worsening mucous, sinus pain, thick nasty mucus, and fevers.  This is the person who is classically described as having bacterial sinusitis who needs to be seen by a doctor.

The person who has sinus pain right after a upper respiratory tract infection with some yellowish, greenish mucous, continued sore throat, usually without a fever, that’s likely viral sinusitis and you will basically just be told to take DayQuil (phenylephrine, dextrometorphan, and Tylenol +\- guanificien).  Maybe some Benadryl to sleep, or some Flonase if you have significant post nasal drip.  But,  all of those meds are over the counter, so you don’t need a doctor to tell you that.

And of course if while having any of these symptoms you notice you can’t move your feet/hands, lose sensation somewhere, have seizures, notice shortness of breath, fevers over 103, pain with inspiration, or just don’t feel like something is right, go see a doctor.  Significant ear pain with or without loss of hearing is another one as ear infection is definitely on the list of things that can happen after a cold.