They will probably never be white again. It could be a product problem since on parts have greyed. Whites need to be washed with whites only. I wash mine with white blouses, no bleach products and they are white as when first bought. So were my panties until I put them in with a bleach product that is suppose to be safe on colors, turned them yellow. I won't make that mistake again. I love white underwear. Another thing is to wear them only one day. Body sweat and oils will stain them.

get some RIT color remover and try that.