Use Retayne which is sold in fabric stores, quilt shops, etc. http://www.dharmatrading.com/chemicals/retayne.html
Use this product to 'fix' dyes in commercially purchased solid colored cotton fabrics or clothing to prevent color bleeding during washing. Also use to add washfastness to your own dyeing. Particularly valuable to quilters. Soak fabrics in Retayne before washing them for the first time. Use one teaspoon per yard of fabric in a HOT (at least 140º F) soak for 20-30 minutes, with enough water to cover the fabric, then wash in hot water and
Synthrapolto get out any remaining, unfixed dye. If you don't do this last step with the Synthrapol, then only wash in cool water rinse and cool water for the life of the fabric. You do risk some fading when using the Synthrapol and hot water, as it will remove any remaining "fugitive" dye.