“Laundry blue" is a solution of synthetic ultramarine that is widely used to improve the appearance of textiles, especially white fabrics. White fabrics acquire a slight color cast after use. Since blue and yellow are complementary colors in the subtractive color model of color perception, adding a trace of blue color to the slightly off-white color of these fabrics makes them appear whiter.
Ultramarine blue is the most commonly used laundry blue & it is also known as " NEEL " in hindi. It gives a violet blue color and makes a fine powder and thus becomes a suitable blue for laundry. It is safe to use as it is not harmful to the fabrics. Bluing is to be done only when the fabric is free from soap or detergent used for washing the cloth. Ultramarine blue being not insoluble in water, the coloring matter is held in suspension, and so the water must be stirred each time before use.