I have always used outdoor sketches and drawings for studio work. I also use photography for detail and composition, but rely on the outdoor paintings for color. Since going digital and discovering Adobe’s Photoshop where I can “diddle” all day adjusting color, contrast and values it became easier to ignore the outdoor sketch. As I was preparing for a 30 x 40 piece in the studio to develop from an 8 x 10 sketch of some large cottonwoods on the Aravaipa River I fussed around with the photo on Photoshop for a half hour only to discover my color ink cartridge was too low. Frustrated, I went ahead and printed out a black and white photo and rediscovered using the 8 x 10 sketch and my own color sense. The black and white photo was a good source for more accurate values and plane separation.
The color on photographs is never very accurate so it’s good practice to rely on the outdoor painting to develop your own color sense and the black and white photo for planes and values.