Here is the start of my raw sienna - burnt sienna turpentine wash. I find that not only does this seal my pencil drawing and give me a value study of the piece, but it also lends a vibrancy to the painting. Some of this underwash will peak through and the juxtaposition of the "orange" wash with cooler hued oil paints adds a natural excitement.
I could use any color for my turpentine wash but I have discovered that this color works with all my nature and wildlife paintings. It is also refreshing to paint on a toned board rather than on white gesso.