Shade with a mix of 3 parts water and 1 part black
Highlight back up with a mix of 3 parts water and 1 part calgar blue. When you laod up your brush tap it on a pc of paper towel before putting it to the model. That will prevent it from puddling.
Add a thin layer of agrax to work as a filter and tone the colrs down.
Use a small amount of undiluted calgar blue to catch the sharpest details.