打底（Underpainting）是艺术用词，经常用在油画上面。打底是作画的第一个步骤，通常是选一个单色在画布上涂满一层（以灰调为主），有助于整个画面的定色。目前有两种不同类的打底技巧，分别是verdaccio（以绿灰色为主）和grisaille（一种灰泥）。

In art, an underpainting is an initial layer of paint applied to a ground, which serves as a base for subsequent layers of paint. Underpaintings are often monochromatic and help to define color values for later painting. There are several different types of underpainting, such as veneda, verdaccio, morellone and grisaille.