Over the internet, it’s very hard for others to evaluate how sick you are. Missing one workout because you’re not feeling well is not the end of the world, but repeatedly begging off workouts may have an impact on your long term goals.
The general consensus is that, if it is a mild head cold (runny nose, sore throat, etc) you should be good to workout –be sure to wipe down the equipment when you’re done–but with a chest cold (coughing up mucus) it’s probably a good time for a rest day or two. There are some other concerns for those that don’t have a home gym. If you are fairly sick you should stay home for the health of others that go to your gym (especially during flu season). If you are sick, can’t get to the gym, and still feel the need to workout try some light bodyweight work (see /r/bodyweightfitness). Keep in mind while you’re sick, you should lower the intensity of your workout to keep the body focused on the task at hand (fighting your sickness) and to prevent injury, as your body overall is in a weaker state than normal.