Often times, dark clothing, especially items made of cotton, appear to be faded and dull.
Instead of fading, it can be a build-up of laundry soap in the fabric. Try adding vinegar to the rinse cycle. Just a quarter cup of vinegar can make a huge difference. Your clothes come out looking brighter and younger.
Also check the laundry soap label for the correct amount to use in your laundry. The lid (measuring cup) is larger than the amount of detergent you actually need.