With my palette laid out with the following colors, titanium white, cad yellow light, yellow ochre, cad orange, cad red light, permanent madder deep (alizarin crimson substitute), ultramarine blue, phthalo blue, phthalo green, and ivory black, I add color to the burnt umber underpainting. I try to get as close as possible to the finished colors, but there's a lot I'm still roughing in. It's better to be darker at this point, so if I need to scumble a lighter color on top, it will be effective.
In my head I hear Daniel Greene's words: "light against dark, warm against cool." I want as much contrast as possible of the tall houses against the lit ones at the back of the alley. I mix a lot of grays. There's not a whole lot of color in this painting. When I was in Germany, I noticed that the red roofs are more like a combo of burnt sienna with a little orange added, as they are the color of red clay.