I always start with the sky and work top down. Cobalt is a favourite.
Then I create a diagonal which will draw the eye of the viewer into the painting to the point of focus - in this case the tractor.
After creating a base layer I add colours to create a depth of texture and variety, generally, darker shades first with lighter on top. Brush strokes are to be seen not hidden.
Finally, I add the touches of lighter and darker shades which bring out the light in the scene, sometimes playing with where light should not be, hopefully achieving a slightly unreal quality to the picture, reminding that it is a picture.