Put simply, wash painting is a hue or tint applied in a thin transparent layer.
Color wash painting
Washes should be applied loosely in the very beginning stages of painting. To create a wash, mix your paint with a thinner such as turpentine. It can be made with acrylics, watercolors, oils or even pastels. If you are worried about the wash blurring your lines, fixative can help set the drawing.