Use soft water for washing hair. Filtered water can be used if the tap water is hard.
Shower heads have minerals and metals which can get deposited on the scalp and hair. So avoid washing hair in the shower.
Wear a hat or bandana while going out in intense sunlight or take an umbrella.
Avoid regular use of chemical-based products.
Try a natural protein-rich hair mask on the hair, once a week.
Massaging with coconut or almond oil is an amazing therapy that can prevent brown strands from spreading through the scalp. It nourishes and retains the black color of hair.
Aloe vera application on the scalp overnight and then rinsing it in the next day can treat hair whose black color has faded.