2 Lay down darks
To fill in all the darker shapes with average dark colours and values, you can either use the same “average” colour from step 1 or alter it slightly, while keeping the tonal values very close.
The best way to decide is to assess your ability. If you are skilled with colour mixing and easily able to keep your mixes clean throughout an alla prima painting session, then I recommend that you do as I did here and go for accuracy and nuance right off the bat with a slightly altered colour. If you prefer to keep it simple, use that first drawing colour to fill in all of your dark shapes.