I've wanted to try this technique for a long time. My husband's great grandfather, who was the head of Croatian railroads, did wonderful watercolor paintings on the backs of train schedules. The paper was a lovely ochre color, and he used opaque white to bring out the light.
So I sketched this little street in the Marais on-site, then painted in my studio at the Cité des Arts. Painted the entire background with yellow ochre, then used grays and chinese white to bring out the lights and darks.
I like this technique, it seems to fit the old and lovely buildings.