Painting Caravaggio's Judith

I have decided to tackle a portion of a Caravaggio painting to better understand his technique.  "Judith Beheading Holofernes" is the painting I chose, and I am going to focus on the head of Judith.  There are a few theories as to how Caravaggio was able to stage and create his dynamic images, but for now I want to focus on the paint application.  Maybe another time I will attempt his full technique, because I think he used the camera lucida, but I haven't had a chance to try it out. 

I am painting over an existing piece that never really went anywhere.  It has a lot of Mars Black on it, which I do not intend to use as my black for this piece. I will be using Ivory Black.  The Mars is VERY blue. What I have learned thus far is:

• Caravaggio's "drawing" is superb in this painting. The face is extremely well rendered. 
• His use of reflected light is beautiful.
• The attention to detail is very interesting, and carefully observed.
• I know he painted with glazes after establishing his form and lights - and that is what I'm doing too.   
• I keep putting down a fairly strong white (with a touch of yellow ochre, burnt sienna and cad red) and glazing over it.  It seems I am getting a decent likeness of paint, but because I do not have an underpainting - or even a decent ground,  I am getting a semi-dull color.  It is tricky to get the warm tones to shine through working off the Mars Black....  I know he painted off a warm ground.