Coloured pencil sketch first (non water-soluble) means you can paint over with thinned out paint while maintaining the drawing underneath. Thinned out brown under the green helps to keep your greens earthy...remember there is earth under the growth anyway!
Thickening the paint on the lights helps them to look even lighter...remember, the light catches the thick paint.
Now that you've seen it, why not try to copy it!