No matter how many times you wash them, older white sheets and towels sometimes refuse to give up their dinginess. Before giving up on them, try subjecting them to this gauntlet of whitening power.
One Good Thing By Jillee suggests washing the offending items as a load of their own, with incredibly hot water. You'll need to set the washer to hot, obviously, but also add in a pot of boiling water for good measure. Then, add a mixture of laundry detergent, dish washing powder, bleach and borax to give the sheets an incredibly deep cleaning. Your sheets should be as white as the day you bought them by the end of the cycle, and if they aren't, then there's probably no reclaiming them.
For the complete recipe, be sure to check out the source link.
Miracle Laundry Whitening Solution [One Good Thing By Jillee]