It seems like every laundry detergent makes claims its product makes colours last longer. According to DIY blog Apartment Therapy, the easiest trick is to add a tablespoon of salt to the load.
The premise is that the chloride in salt helps keeps clothes bright by sealing in the colour across repeated washings. We’ve already seen salt’s power to remove bloodstains, but it turns out it’s a great protector of clothes as well. If you have some brand news clothes you’d like to protect, How Stuff Works recommends using a 1/2 cup of salt to prevent colour bleeding on the first wash.
Use Salt to Keep colours From Fading In Your Fabrics [Apartment Therapy]