If you have not dryed the sweater in the dryer and if the sweater can withstand hot water, then Synthrapol may work for you. If you dried it in the dryer, then the color is probably heat set and won't come out. I have used Synthrapol a number of times on quilts and clothing, and the color has been removed -- it may take several washings. This is a staple on my laundry room shelf.