you could try spraying the white (dingy) areas with shout or spray & wash- then wash it separately- which should remove what has caused the dinginess- or oxyclean laundry treatment.
retayne sets dyes- synthropol suspends fugitive dyes into the water & keeps them from getting on other fabrics- too late for either-
i would use a stain remover & maybe toss in a color catcher. I've used shout many times to (clean/brighten up) clothing items that have had something bleed on them (like turning hubbys socks pink when they went in with a red top i though would not run because it had been washed many times...spraying the socks with shout & rewashing made them white again.