I washed a white sweater with blue letters with darks and now it is a dingy white. How can I get it back to a bright white without discoloring the blue letters?

Normally a bleach soaking solution (1/4 cup Clorox^® Regular-Bleach bleach per gallon of water, with a soak time of up to 5 minutes) will strip away dye transfer from fabrics that can be safely bleached--note that you should always avoid bleaching wool, silk, mohair, leather, and spandex. The big challenge on your garment is the blue lettering, which is likely highly visible without any hidden areas. This makes it difficult to test to see if the blue letters could survive the bleach soak without risking a white spot if the fabric fails. Here’s the test for bleachability: Add 2 teaspoons Clorox Regular-Bleach to 1/4 cup water. Apply a drop of this solution to a hidden part of the item and wait 1 minute, then blot dry with a paper towel. No color change means the item can be safely bleached.