Below is a grid of four small color pastel studies I did based on a photo. (These images are from an exercise I did with a student yesterday; you can click on the image to see it larger.) The top left photo (#1) shows a sketch that I did in full-color directly from the photo. This is how most of us, I suspect, paint from photos. I tried to analyze the color as best I could. It was OK, but the color is a bit ho-hum.
In order to find a process that would make the color more exciting, I stepped back a bit to basics. First, in #2, I did a simple value sketch in four values. Next, in #3, I analyzed my value sketch and selected cool colors for the two dark values and warm colors for the two light values. I purposely selected colors that I didn't see in the photo. Finally, in #4, I made a copy of #3 and then used the colors I did see (the "local color") to layer over these. In my mind, #4 has richer, more interesting color than what is happening in the cut-and-dried #1.
To be sure, this is a gimmick - but sometimes coffee and rock-and-roll just aren't enough to liven up a rainy studio day!