When painting in oils and acrylics, not watercolours, I always paint the entire area in the darkest colour first. For example bushes or tall grass and weeds have a common dark colour between the leaves or blades of grass. This colour may vary from dark green to almost black. (I never use black paint) After the entire area is painted, I add the other colours but leave spaces in between allowing the dark background colour to show in between the leaves or grass.
In oils I form leaves or grass over top of the dark background colour. This is done by painting lighter shapes over the dark to represent grass or weeds. With my brush I carve the paint over the background the same way you would carve leaves out of clay.
