Vermilion, the color for ex-libris ownership stamps, has been used extensively in both China and Japan for centuries. The reason for this is that "vermilion ink" has long been considered the counterpart of black "India ink." This vivid color also presented a strong contrast with the black used for printing, calligraphy, and drawing. Unlike blue or green, there was no fear of it fading or changing color with age.
Vermilion has long been considered a royal color in China. As it did not fade or vary in shade over time, it was held to be unchanging like gold, making it an auspicious color sought after by people seeking perpetual youth and long life. And so it was used as a pigment for ink pads. It is well known that vermilion was expensive and has been highly valued in Japan since ancient times. As vermilion was being used in a culturally advanced country like China at that time, it seems natural that the people of Japan would follow suit.
Ex-libris ownership stamps use vermilion ink pads as their source of color. The formula for these can be seen in "Wakan Sansai Zue" (encyclopaedia) edited by Ryoan Terajima, and "Mojuno no Kusaguki" by Nanpo Ota. There are slight variations in these materials, but the basically accepted method is to grind cinnabar into a powder, then add some oil, such as castor oil, little by little, while mixing thoroughly. Following that, wormwood was carefully chopped to produce a cotton-like texture and then added to the mixture. Alternatively sesame oil was used.
A list of formulas used in China can be found in "Ju-jia Bi-yao Shi-lei Guan-ji" written by an unknown author during the Yuan dynasty. In China, the upper classes used an ink pad made by adding precious stones, gold leaf, pearls, coral, agate, or isinglass to give it an iridescent color.