This is a tannin stain - a plant component that often shows as a color in the final product. Fresh tannin stains can usually be removed by laundering the garment using detergent in the hottest water recommended for the fabric.
Never use natural soap in a bar or soap flakes because they make tannin stains more difficult to remove.
Older tannin stains may need to be treated with bleach. Chlorine bleach can be used on white garments and linens. Use color-safe bleach (OxiClean, Clorox 2, Country Save Bleach, Purex 2 Color Safe Bleach) for colored fabrics.
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