Wikipedia
Imprimatura is a term used in painting, meaning an initial stain of color painted on a ground. It provides a painter with a transparent, toned ground, which will allow light falling onto the painting to reflect through the paint layers. The term itself stems from the Italian and literally means "first paint layer". Its use as an underpainting layer can be dated back to the guilds and workshops during the Middle Ages; however, it comes into standard use by painters during the Renaissance, particularly in Italy.
The imprimatura provides not only an overall tonal optical unity in a painting but is also useful in the initial stages of the work, since it helps the painter establish value relations from dark to light. It is most useful in the classical approach of indirect painting, where the drawing and underpainting are established ahead of time and allowed to dry. The successive layers of color are then applied in transparent glaze or semi-transparent layers.
Care is taken not to cover the imprimatura completely allowing it to show through the final paint layers, this is effective in particular in the middle to dark shadow areas of the work.
An imprimatura is usually made with an earth color, such as raw sienna, and is often diluted with turpentine. The term imprimatura should not be mixed up with the term imprimatur, a term used in editing.