I love a good pair of dark wash denim jeans. They are practically all I wear. Keeping them the same color as when I purchased them is another story. I have worked long and hard to find the best ways to keep them as dark as the original without my hands still turning blue while wearing them.
Turn your denim inside out, and only wash denim in the load, no other clothing
Use cold water on a gentle wash cycle add mild detergent plus 1tsp white vinegar per pair of denim in wash
Vinegar helps set the dye, is a natural cleaning agent and odor remover
Also try a specialized detergent for darks
Add fabric softener for fresh smell and to soften clothing
Lay out until partially dry then on low heat, do not over dry will fade color
Wear denim 3 times before washing again
Hope these tips help.