Towels should be washed in hot water with detergent. Hot water is preferable to cold water because the high temperature helps to kill bacteria and removes oils from the fabric.
The use of hot water, detergent and color-safe bleach greatly improve the cleanliness of towels. Cold water is better to prevent shrinking and wrinkles, but these are not major concerns when you are washing towels.
Switch to cold water for the rinse cycle for the towels. This gives you all of the benefits of using hot water for the wash cycle, it saves a little electricity and it reduces wrinkling.