A cold is caused by one of a couple of hundred viruses, most commonly rhinoviruses. Understanding which symptoms come from the virus and which from your response explains a lot about what helps.
The symptoms are mostly you
The virus infects cells in the upper airway. The immune response to that infection produces the inflammation, mucus, swelling and general awfulness.
That is why symptoms appear a day or two after infection rather than immediately, and why they persist briefly after the virus is cleared.
Why antibiotics do nothing
They kill bacteria. A cold is viral. Taking them does not shorten a cold, does cause side effects, and contributes to resistance.
Green mucus does not indicate bacterial infection, contrary to common belief. It reflects immune cells doing their job.
What has some evidence
Rest and fluids, which support the response rather than fighting the virus. Paracetamol or ibuprofen for symptom relief. Saline rinses for congestion. Honey for cough in adults and children over one year, which outperforms several cough medicines in trials.
What has weak or no evidence
Vitamin C taken after symptoms start. Most over-the-counter cold remedies, which combine ingredients at doses that do little.
Zinc has mixed evidence and side effects that many people find worse than the cold.
When it is not a cold
High fever, difficulty breathing, symptoms worsening after initial improvement, or duration well beyond ten days warrant seeing a doctor.
The caveat
General information rather than medical advice, and anything concerning should go to a doctor rather than an article.