You cannot alter a watercolor once the paint is on, nor can you layer paint. While they are difficult to execute well, however, watercolors lack the spatial depth and power of oils.
水彩畫入門容易，但畫得好、自成一家並不容易，而且水彩不能改、不能堆疊，層次和氣勢比油畫相對地要弱。