Especially dark jeans first time to buy the water, it needs to pour some white vinegar or salt, at the same time the pants turn over soaked for about half an hour, used to lock the color.
Since dark pants are bound to fade a little, white vinegar helps keep that deep blue color as shiny as possible. Please do not use hot water soaked pants, that will have a large degree of shrinkage, the general temperature of water in 45 degrees or so can change.
If the conditions allow, please do not use the washing machine to wash jeans, that will destroy the drape of the pants, the color of the pants, the body of the natural white will become unnatural.
If you use white vinegar, pour it into cold water and soak your jeans in the water for half an hour.
The same goes for salt