Double-loading a brush has the effect of blending two colors together as the brushstroke is made.
• How to Blend Paint Colors
• How to Load a Flat Brush with Two Colors
By Marion Boddy-Evans, About.com Guide
In this detail from a pastel painting, the colors on the forehead have been blended to give a smooth skin tone.Photo © 2010 Marion Boddy-Evans. Licensed to About.com, Inc.