Tonal Underpainting - Sparrows

I experimented with a slightly different technique today.  Typically, I start by "drawing" the subject of my painting, using a small brush and burnt umber paint, either undiluted or diluted slightly with water (my paints are water-miscible).  This gives me a fairly detailed line drawing of the subject.  Because I then start immediately adding color, I have to make tonal decisions (lights and darks) at the same time as I am trying to make color decisions.  Today I decided to do a complete tonal underpainting before adding any color.  This is kind of similar to the "grisaille" technique used by some of the old masters and many artists today, where the artist does a complete tonal painting in greys before applying color.  I am not sure whether I should add solid layers of color over it or try some thin glazes.  I'll find out tomorrow!  (The painting is 16x16" - click for larger view).