If your dark-coloured clothing tends to get white stains on it, there could be several causes. Unlike lighter coloured garments that are more susceptible to stains from obvious dark culprits such as protein stains, dark clothing is much more likely to hide dark stains but make white stains very obvious. Avoiding these causes or pretreating the stains can keep your dark colours looking good long term.
Antiperspirants contain aluminium salts that keep you from sweating, but unfortunately this substance also causes white stains to form on dark clothing.
The salts, such as aluminium chloride, aluminium chlorohydrate or aluminium zirconium, combine with the electrolytes in the sweat to form a gel. This gel clogs the pores where sweat is secreted and stops the sweat from reaching the skin's surface for a period of time. The salt in the sweat and in the antiperspirant can leave white marks on dark clothing where it comes in contact. Usually these stains are found under the arms of shirts or dresses. Underarm stains can usually be removed in cold water or by treating the stain with ammonia, aspirin, baking soda, lemons or vinegar.
Toothpaste can leave stains on clothing. Usually these types of stains are easily removed using plain water after the toothpaste has dried. Wash these clothes as usual with your detergent and the stain will likely be gone. Pre-soak the stain in water and rub it with your fingers as an additional precaution.
Powdered detergents for laundry are meant to clean your clothing and remove existing stains, but sometimes this powdery substance can actually cause stains on dark clothing. This is a problem for people with hard water. Detergents are unable to dissolve completely in hard water because of the high mineral content of the water. Use the hottest water that is appropriate for the material to reduce the chances of staining by increasing the amount that dissolves in the wash. You may also choose to purchase a water-softening additive to help with the detergent's ability to dissolve thoroughly.
Sweat contains a high concentration of salt. Just as the salt from antiperspirants can stain dark clothing, excessive sweating can leave a ring of white salt on dark clothes. Often you'll notice dark coloured shirts with white rings around the collar after profuse sweating. Usually this salt will wash out during a normal wash cycle, but you may wish to soak the material is cold water to dissolve the salt if the stains are tough to remove.