Hi Angharad M,
I'll be honest with you, I know very little about the practice of yoga in it's physical forms. Though the breathing exercises that are part of pranayama yoga can be beneficial in addressing cold symptoms; especially one nostril breathing.
However, if you practice more physical forms of yoga I would advise you to listen to your body and rest. If you feel like practicing gentle yoga exercises; then do. You know your body the best. Eat lightly, avoid dairy products and drink plenty of fluids e.g. simple herbals - fresh ginger root, mint, fresh lemon slices and honey. Without knowing your constitution and cold symptoms it would be unprofessional to suggest a more specific herbal remedy.