How I Paint Fast Changing Light Effects

There's this old farmhouse in my town that has long been abandoned and new development is encroaching upon it.  There are many paintings I want to make from this subject but of particular interest is how, at sunset, the pine tree next to the house casts cool purple shadows on the warm red brick.  The problem is that this light only lasts 30 minutes at most and a camera isn't able to capture the colors.   I could do some color studies in the field and paint the finished piece in the studio....but I wanted to paint the entire picture en plein air.

So, how do I capture a fleeting light effect en plein air? 

Pre-work: I went to the location several months ago and did a series of sketches in my sketchbook to work out possible compositions.  At home I decided which one I liked best and further developed the value structure in pencil.  With the composition decided upon I waited for a night with clear skies and potential for a nice sunset then went out and set up to paint.

Painting:  I arrived an hour before the light I was looking for so I had time to set up and do a detailed drawing on my painting surface of the composition I had worked out.  Next, I mixed some piles of local colors to correspond with the major elements (sky, tree, house in light, house in shade, foreground grasses).  The colors would still need some modification to match the effect of the sunset lighting but the relative values of the piles were accurate to my vision of how I wanted the painting to look and this is most important.  These pre-mixed piles help me to paint faster because my starting point is almost already a match.  It's important to adjust color without significantly changing the value of a pile...otherwise you risk messing up the underlying value structure of the painting.
As soon as the sunset light was right and the cast shadows from the pine tree were in place I started painting.  I adjusted colors and painted patches in the sky and building...focusing on the relationship of sun and shadows on the building.  I didn't try to finish, just get the major shapes in and color relationships correct.

Wrap-up:  Soon, the sun was behind the mountains and the scene had changed from my concept.  However, the sky remained light enough for another hour to keep painting.  At this stage I focused mostly on the painting and completing the colors and patterns I had put down during sunset.  I only refer back to the actual subject for information on details like windows and trim.  As it became dark, I set up my book light which allowed me to finish the painting.  My book light provides a nice white light that doesn't add a color cast to the colors.  This little light enabled me to stay the extra 30 minutes and complete the painting.

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