Under Painting in Oils
I do not always use the indirect method of doing a monochromatic under painting in oils preferring in most cases to work more directly. However, I've always found that when I do make use of an under painting I am never disappointed. There is an unmistakable beauty in looking through an over painted glaze or a translucent scumble of color that adds a richness to your work that's not as easily achieved in working directly. One looks "into the color" as opposed to "onto the color". And if the color one achieves is not right it may be "unmixed" by wiping off the top pigment and trying something else.
Normally it is better to paint the under painting in a higher key so that the over painting can benefit most fully from the glazes and scumbles. Glazes are transparent in consistency and are transparent colors that are made more transparent by adding medium. They add color to an under painting but also darken the underlying value. A scumble is a more opaque pigment that is made translucent in consistency by adding medium. It adds color but actually lightens the underlying value. I maintain that if one can darken a light with color and lighten a dark with color than the sky is the limit and a painting with a good composition should never seem beyond hope!
Here is an example of under painting which shows on the left half of this composition. It was painted using ultramarine blue, transparent oxide red and titanium white. The entire under painting was painted in a higher key keeping the values simple but relative to each other. The right half was under painted just like the left half but after drying the right half was glazed and scumbled with various colors. If you have never tried it I think it is well worth the effort and several under paintings can be begun at once.