Michael, I use a variety of photos to put together a design for scratchbuilt flats. Torso here, arm positions there, different head tilts. Sometimes it works, sometimes it doesn't. Photoshop helps. When I do the drawings I can add costuming and different hair styles. I make the drawing about twice actual size, then scan and size it to the scale/size I want in Photoshop, print it, then transfer the drawing to sheet plastic which I use as a foundation for the putty. Since I control the light direction I just paint, shade, and highlight as I see it in my head. I do use a color wheel to map out paint colors and keep a list in case I have to make touch-ups or repairs later.
Cheers,
Glen