You may think the answer to this question would be straightforward. The truth is, you need to listen to what your body is telling you as well as what your doctor is telling you.
Here are some questions to ask yourself:
Can you sing?
Before you ask if you should sing, you need to first determine whether or not you have the capability to sing. Have you lost your voice or does your voice seem normal except for some excess phlegm? Not sure? Do a light warm-up using several humming scales. Stay in the comfortable part of your range until you feel your voice is solid. If it feels good, then do a few exercises going into your higher range.
Sometimes when you have a cold you need a longer period of time to warm up. There will be times, however, when your voice will not improve even with a warm up. Those will be the times when it’s best not to push yourself. If you have lost your voice, then you are unable to sing. The best you can do for your voice is rest. Try not to talk; write notes instead and definitely do not sing.
How is your energy level?
Let’s say you’ve determined you can sing. Next ask yourself how long you will last. Can you walk across the room without running out of breath or getting tired? Could you walk down to the corner market? If you will only last 15 minutes then collapse for 2 hours to recuperate, is it worth it? Will you have accomplished anything during your practice time in this instance?
What kind of singing are you doing?
Are you practicing, attending your lesson, or doing a rehearsal or performance? Does your performance include movement or dance? How long will it last? How difficult is the repertoire? Can you handle it with reduced capacity?
Most people wouldn’t hesitate to reschedule a lesson or practice time unless they are preparing for an upcoming performance. I suggest to my students that if you can sing and your energy level is ok then we should have your lesson. There may be a day when you need to perform with a cold. Having a lesson while having a cold can help prepare you for that. Most singers would need to make adjustments to their breathing, phrasing, movements, etc and having their instructor there to guide them and prepare them is an asset.
There is no shame in changing repertoire or dropping songs from a set because you are sick. It is better not to test your threshold while you are under the weather; it is an easy way to cause damage to your voice. If you choose to sing, play it safe.
Overall, please be aware that if you are taking OTC meds to reduce your cold symptoms, you need to drink extra water to stay hydrated (especially if you plan to sing). Keep in mind that if you have a sore throat you should not sing. If it hurts to sing or speak, then you should not sing or speak. If you use menthol/eucalyptus cough drops to numb the pain, you should not sing. If you numb the pain then sing, you run the risk of hurting yourself more. Consider if you broke your ankle, numbed it, then ran and danced on it anyway.
Always consult your physician. I am a singing instructor, not a doctor. Your doctor has precedence over me.