Using the space in a painting can create some great effects with very little effort.  For example, see the bush above.  It reflects the light and dark effectively.  This was achieved by applying black gesso and allowing to dry.  I then used the Bob Ross paints and took a kitchen scourer to add a thin layer of Liquid Clear.  On top of this, was sap green dabbed on with a brush.  Then all that was used to complete the process was cadmium yellow mixed in with liquid white.  This was applied with a fan brush, but I left plenty of gaps so that some of the sap green and parts of the original gesso showed through.