Shout Color Catcher Sheets for Laundry, Maintains Clothes Original Colors, 72 Count

Yes, softsoap is a well known culprit. As is Dial. Best alternatives are 7th Gen, Dawn Ultra (many folks I know dilute it with water in a 1:1 ratio), Mrs Meyer's, or Dr. Bronner's. You really should consider adding a booster like Biz or FEBU, going with warm as your default temp setting and also using color catchers.