A cold weakens your body's immune system, and takes time to fight off. Being outside in cold or wet conditions can also tax your system, prolonging or worsening the cold.
That said, a little sunlight and fresh air on a mild day can do just the opposite.
Also, contrary to popular belief, it isn't usually the cold weather that causes the common cold - it's usually all the added time that we spend indoors during that season. Dusty heater vents, close proximity indoors to a lot of other people, ect., do a lot more to spread the cold bug than a few snowflakes ever did.