I tried my experiment again- to paint from a black and white photo, and using my own choice of colors. My palette was again limited to white, ultramarine blue, and cadmium red and cadmium yellow, but this time I added lemon yellow so that my greens could be more varied. My theory is that if the values match, you can choose your own colors and the painting will work.
I find it a real struggle to paint clouds. I ended up changing the sky quite a bit from the photo just to make it work.
After I finished the painting I checked to see the reference photo in color and was surprised at how much more saturated I had made the colors. Here is the image I looked at while painting (from the free photo sharing site Morguefile):
and the image in color:
You might wonder why I was drawn to such a moody image. Well, I liked the moodiness, plus really fancied the dark values of the land as compared with the sky, cutting into the composition like the wedge of a knife.