Choose colours lemon yellow, raw sienna, prussian blue, neutral tint

the neutral tint will give a very strong dark colour

atmo tree1   atmo tree2   atmo tree final
Roughly sketch the composition, then spray or paint clean water on the whole paper. Paint on raw sienna in the middle section above the horizon line while the paper is still wet. It will look very dark but will dry 30% lighter. You can drop in more colour while the paper is wet, BUT once it starts to dry results can be unpredicatable.  

Note the pale area it's important to keep it light. While the paper is still wet drop in prussian in the sky and green with prussian and lemon in the foreground add a little raw sienna at the bottom for balance.

 

  If the paper is still wet add more colour if the paper is starting to dry then, dry the paper completely, the paper should be flat,then wet the paper again and add more paint.
sloping tree final  

Put in the foliage first - Let your brush skip over the paper using some dry brush strokes on the edges

Avoid painting the trunks too thick (you can always make it thicker if too thin but the opposite is much more difficult!) Don't forget the "bird holes" Draw the branches over the white areas with a rigger brush