It depends from the cold and the symptom you want to alleviate. I know and use some of them:
Ginger is a classic. Good for a sore throat and if you have issues like nausea. I cut fresh ginger and cover it with honey in a clean jar. It stays good for one year. one tablespoon in a tea or other infusion and the cold potion is ready.
Elderberry syrup is also a good plant together with willow bark tincture. Dosage of both should be monitored and the active principle concentration in the plant or the final product should be known. Those remedies are good if you seek relief from pain associated with cold and have no nausea or stomach issues.
Licorice is a good plant if you have a cold with cough. I bring 2 cups of water to boil with 2 tablespoon of licorice roots at low-to-medium heat. Over the tie of 2 hours the concoction is reduced to half a cup and that concentrated extract will help with the cough if taken twice a day.
Over the concoction I also take 5 to 10 drops of juniper essential oil on a teaspoon of sugar.
Hedera or Ivy is a poisonous plant that is given in exact doses to stop the cough. Although it does not cure it, it helps if you can't sleep at night because of it. Home-made remedies can be dangerous!!
Thyme - as form of tincture to alleviate spasm of lungs, stomach and gut.
Woodspider - a herb with low toxicity also useful to cure inflammations. For a cold you can use a tea made of 3 to 5 grams 3 times a day unless you have problems with your stomach, gal bladder or you start having diarrhea.
Frankincense as form of extract or tincture is a potent anti-inflammatory plant. It works even better if mixed with licorice and/or willow.
Eucalyptus - in a form of tea at 3% concentration. useful when you have mucus and/or cough.
Tilia flowers (should be known as lime three, linden or basswood) also it helps to alleviate the symptoms associated to the cold. It has a relaxing and sometimes narcotic effect so is better to take it before going to sleep.