Does green mucus or phlegm mean my child needs antibiotics?

What the evidence says

It's very common for a child's runny nose to change from clear to yellow or green partway through a cold - this is a normal part of the immune response, not proof of a bacterial infection. The Canadian Paediatric Society notes that thick or coloured discharge only becomes a reason to call the doctor if it lasts more than 10 to 14 days. Choosing Wisely Canada is explicit that coloured mucus does not necessarily mean your child has a bacterial infection, and that antibiotics do not help most respiratory infections.

The American Academy of Pediatrics guideline for bacterial sinusitis agrees: diagnosis is based on how long symptoms last (10+ days without improving), whether they get worse after initially improving, or whether there's a high fever with thick discharge for at least 3 days in a row - not on colour alone. So instead of watching mucus colour, watch for these red flags: symptoms lasting more than 10 to 14 days without improving, symptoms that get better then suddenly worse again, a high fever (39°C or higher) together with thick discharge for several days straight, or signs of an ear infection like ear pain or drainage. Any of these are reasons to check in with your doctor.

Most colds, including ones with green snot, get better on their own within one to two weeks with supportive care like fluids, rest, and saline drops - no antibiotics needed.