Barbara,
Good luck to all of you. When you gotta sing and you have a cold, treat the symptoms. Don't buy one of those shotgun remedies. Think about what's interfering with your singing. If your nose is dripping on your guitar, you need a decongestant. If you throat is all mucousy, you need an expectorant. If your throat is sore, ease it with warm tea. Don't use any mentholyptus cough drops. They are much to severe. If you are coughing, try a cough suppresant.
Let us all know how it goes tonight! And remember, you always sound better than you think you do. I did an open mike last night. I felt it was only a fair job, but I received some compliments and many encouragements to come again!
Roger in Baltimore