Why are ancient landscape paintings mostly black and white?

Ancient landscape painting was originally based on green landscape with pigments, such as You Chuntu in Sui Dynasty and the "A Thousand Miles of Mountains and Rivers" by Wang Ximeng in Tang Dynasty. Wang Wei is regarded as one of the advocators of ink and wash landscape painting of "Southern Sect". In the Northern Song Dynasty, there were ink and wash landscape paintings handed down from Mi Youren. Since Yuan Sijia, ink landscape painting has gradually replaced the dominant position of green landscape painting. It uses five colors of ink instead of pigment, and pursues the artistic conception of literati seclusion, which is called "my realm"