Dark's in watercolor require a knowledge of viscosity or the amount of water used with the pigment. This is a great skill and a requires good practice.
For this assignment begin with:
I will be using the Old Masters Limited Palette of Yellow Ocher, (my yellow), Burnt Sienna, my red) and Paynes Gray, (my blue). All other colors greens, oranges, and violets will be mixed from these three primary colors. Yes the color harmony is very muted (low intensity).
Step by step examples are forth coming.