I wanted to keep this painting an exercise in painting a darker low keyed job, no big lights or whites very subdued and quiet. I used the complements of each color while retaining the color. What that means is you add just a little ultramarine blue to the burnt sienna, keep the burnt sienna feeling but not intense. For the greens I would add a violet or alizirin crimson but again I want it to read as a green. Learn your color wheel, that is the colors on your palette and how they go together. 
Raw sienna, cobalt blue, cobalt violet and olive green for the sky and carry these colors into the the distant horizon. I will use these colors a lot in the rest of the painting.
Burnt sienna, olive green, raw sienna for the field, let the colors merge on the paper, Oh and like um, I forgot, the distant blue hills are cerulean blue.
Dark rich burnt sienna, ultramarine blue and alizirin crimson with a hint of green make the foreground. Use the side of a big round to apply the paint. Scratch out some details with a knife.
Cobalt violet, burnt sienna and a touch of ultramarine blue for the barn. Remember to get some variety in the color we are not painting the kitchen.Details and finish. Keep control of yourself and vary your brush marks. Too much repetition creates boredom.