I have started the grizaille and 'am discovering all sorts of challenges. First off thanks to the wonderful back light and secondary light coming from her left the values I get to work with are quiet an exercise in restraint.
The background is the lightest part of the painting and leaves a value range from White (0) to no darker than value 7. This leaves very little room for modeling the curtains.
Tiffany on the other hand is in the middle ground and starts with highlights at value 7 and going somewhere down to value 4 on the skin. That secondary light source is just about killing me, because it makes the whole thing rather confusing. The trick here is to get Tiffany to stand out from the background without making her too dark. Needless to say... I am having fun!
Foreground is kept really dark. Highlight there are no lighter than value 6 with most of the values staying around 4 and 3. I'll try to shoot a better photo without glare, once everything dries a bit.
Right now with the grizaille the aim is to get all the values dead on. This also means that extra care is placed upon lost and found edges. Since I have a tendency to paint rather thin, I decided to cover every inch of the canvas with rather heavy paint, using absolutely no medium.