I've had luck with elderberry preventing a cold if I take it right at the first symptom, but if I'm already sick it doesn't do a thing.
I have found taking Vitamin C at the first sign of a cold also works. As C is used up or excreted in 4 or 5 hours, you have to take it 5 or 6 times a day. Rose hips are a natural source of C. When plague was ravaging Europe I read that Nostrodamus urged his villagers to make rose hip tea, and they had little plague.
At a wilderness campout, a young women stung by yellow jackets had forgotten her allergy kit. Her skin was swollen an inch all over, her eyes were puffed almost shut, and her throat was closing up. I started giving her Vitamin C, ten grams at a time, every 1/2 hour. At the first 1/2 hour, her throat had opened a little and she could see better. At 1 hour she felt a lot better, was no longer afraid she would die, and about half the puffiness had subsided. When I had given her 40 grams, she was a lot better and I cut back to 5 grams per hour. I stayed with her six hours, and then she said she felt she could take care of herself and take C as needed.
Another powerful antioxidant is Pycnogenol. It is expensive but I keep some around for emergencies. You can probably get it naturally by brewing young stem ends of Arbor Vitae, the ornamental evergreen shrub, into tea.
Natural vinegar (apple cider vinegar), saurkraut, kimche, etc. all seem to have useful properties. When my children were very small I would fill an 8 oz., baby bottle with water, a tablespoon of honey, 1/2 teaspoon of garlic powder, and 1,000 mg. of Vitamin C, crushed into powder. They would suck on it during the night when they woke up coughing, and go back to sleep. After I started doing this they rarely got ear infections from colds.