OK so firstly in an ideal world you would not sing with a cold and would rest, but if you have to and you have a performance, there are some things you can do. The key is to get the mucus out so it’s really important to stay hydrated. The main reason being because when you have a cold the mucus becomes dry and sticky but when you’re hydrated it becomes warm and wet and can then move out of your system more easily.
Steam inhalation is also another great thing to do. This will help to reduce the swelling of the mucus membrane lining. Steam inhalation will immediately relieve some of that swelling and you can add eucalyptus oil to the steamed water too to really clear things out. This video provides some other natural remedies.
The main don’t is to avoid over-the-counter medicines like cough suppressants, decongestants and antihistamines as these will just dry out the mucus which then makes it harder for the body to get rid of it.