Along with orange and its opposite, purple, red is the color at the long-wavelength end of the visible light spectrum. It has a dominant wavelength of about 625-740 nanometers. It is a primary color in the RGB color model and a secondary color (composed of magenta and yellow) in the CMYK color model, and is the complementary color of cyan. Reds range from bright yellowish scarlet and vermilion to bluish crimson, and range in hue from pale pink to deep burgundy. Ocher-based red pigment was one of the first colors used in prehistoric art. Ancient Egyptians and Mayans painted their faces red in ceremonies; Roman generals painted themselves red to celebrate victories. It was also an important color in China, where it was used to color early pottery and, later, the doors and walls of palaces. During the Renaissance, the bright red costumes of the nobility and the wealthy were dyed with kermes and cochineal. The 19th century saw the introduction of the first synthetic red dyes, which replaced traditional dyes. Red has become a symbolic color of communism and socialism; Soviet Russia adopted a red flag after the 1917 Bolshevik Revolution until the collapse of the Soviet Union in 1993. Communist China adopted the red flag after the 1949 Chinese Revolution. It was adopted by North Vietnam and throughout the Vietnam in 1954. in 1975. Red is the color of blood and has always been associated with sacrifice, danger and courage. Modern surveys in Europe and the United States show that red is also the color most commonly associated with heat, activity, passion, sexuality, anger, love and passion. In China, India and many other Asian countries, it is the color that symbolizes luck and happiness.