Pigment blue is one of the oldest pigments known to mankind. It is widely used in food, medicine, printing, and dyeing industries. The application of indigo as a fabric dye can be traced back to at least 2500 BC. Some clothes worn by ancient Egyptian mummies and blue linen fabrics unearthed from Mawangdui in China are dyed with indigo. A branch of Yao nationality in China is named “blue indigo Yao” because of its unique technology of producing and using indigo-dyed cloth.