If you feel like lifting weights when you have a cold, then give it a go. if you feel like complete rubbish, it is better off that you get rest. My advice would be not to plan on breaking any PRs. Instead get into the gym and just start moving some lighter weights around. If you feel yourself getting a bit of a pump, and the endorphins kick in, then keep going. Don’t go crazy, the aim of this workout is to make you feel better, not worse. Alternatively, if you feel nothing and fatigue quickly it is time to abort the mission. Your body is not up to it. Go home and rest. Don’t be down on yourself, you gave it a shot. The goal with any illness is to recover as quickly as possible. If you have a busy schedule and you are training hard, your body may have been overworked and the reason you became ill. Don’t feel bad about taking it easy. Look at it as a way of giving your body a well earnt break. Once you feel better, you will be back in the gym, fresh and eager to get back into it.