You may think that is a very simple task but a lot of people still do it wrong. Take a look at the chart below to determine the effects of cold and hot water in the hair. Conclusion: You should wash your hair with hot water to remove product build up and dirt. Rinse out your conditioner with cold water to seal the cuticles and create a smooth finish that will have your hair looking shinier. If you have color treated hair washing your hair with cold water will help prevent fadeage.