By adjusting the color of black with the other colors, I am able to warm or cool the areas of the dog that are either in the sun or in the shadow. If you look down at his feet you can see the masking fluid. I also masked out the spiky hairs on his chest where the light was hitting it the strongest. Far easier than trying to paint around those clumps of hair!
One other trick I've learned - especially when working on a painting that has values as high as 9 or 10 - is to go ahead and put in the darkest dark to be found in the painting. This then gives me an area by which I can judge all my other values. It is sometimes difficult to do because it looks soooooooo dark against all that white paper, but that's where trusting your painting abilities comes in to play!
If you are really unsure of how dark to go, you can 1) go dark in just a small area 2) make a black and white photocopy of your reference and use a value scale to make sure you have the right value, or 3) chicken out and go darker in layers. The layers can be a very effective and beautiful way to accomplish darks. And that does it for today!