First of all, there are over 200 viruses that can cause colds, but some of these, like rhinoviruses (which are the most common cause of colds), tend to peak in the autumn, whereas enteroviruses are more common in the summer. Enterovirus infections can cause more severe symptoms, and unusual symptoms such as rashes.
That being said, despite the myth that summer colds are caused by enteroviruses, this family of viruses can cause many different ailments, but remains a rare cause of summer colds themselves.
The main reason for the difference between summer and winter colds may simply be psychosomatic –- in other words we probably feel like summer colds are worse because we think we’re not supposed to catch a cold in the summer.