Using a camera obscura for this directly does not actually work for color paintings. As soon as you start painting you get the color projected atop your paint,

Using a camera obscura for this directly does not actually work for color paintings. As soon as you start painting you get the color projected atop your paint, making it essentially impossible to match the colors. You need a small modification with a mirror so you can see the projection "next" to your painting instead of atop it. Tim's vermeer is a highly amusing documentary on the topic. I've got no real clue (or stake) in its accuracy, but it's worth watching for the idea alone.