This clip shows work on one of the earlier stages; the underpainting. Basically, it's the entire painting done first in shades of thin gray paint (sometimes green). After it has thoroughly dried, the final layer of color is applied. This is an old technique; most of the classical dutch masters worked this way, for instance. It's also time-consuming, probably adding 30-40% to the execution time for each painting.
But for me, at least, I think it contributes to a stronger final result. A well-done underpainting provides a clear map of the values (degrees of lightness or darkness), essentially solving one big problem before color is involved (seeing and mixing accurate color may be the single biggest difficulty in painting). It also provides a good foundation for transparent colors. Some colors (most violets, for instance), are so transparent that it becomes quite difficult to establish a darker shade if applied directly on white canvas. With an underpainting, however, that problem disappears - the proper value is already there, and the pigment simply becomes a glaze to state the color. Less concretely, an underpainting can act as a dress rehersal for the final painting.