This video art tutorial done by Sessions Online School of Fine Arts teaches you how to do an underpainting for portrait painting.
http://www.tvlesson.com/video/36294_portrait-painting--underpainting.html
This video tutorial will teach you how to do portrait painting by underpainting color layer. What you need to aim here is achieving 80% accuracy to what the finished painting colors would be and do...
http://www.tvlesson.com/video/36299_portrait-painting--oil-painting-technique-color-layer.html
