Dark clothes should not be washed in warm water to avoid causing the colors to fade. It is best to use cold water, since warm water promotes fiber breakdown.
The type of water that is used can also cause colors to fade. For example, hard water that is rich in magnesium and calcium can cause damage to clothing over many washes. Hard water can also lead to decreased effectiveness of some detergents. It is best to use liquid detergents, which dissolve more effectively in cold water, and to utilize products that are specifically designed to clean clothes in cold water.