There are two kinds of water-color artists: those who use white paint and those who are going to use white paint. Just kidding. Actually, there are quite a few watercolor artists who absolutely refuse to use it, and I support them. I even, on occasion, refrain from the good old Titainium. There is an unusual beauty to the clean, pure expression of a transparent watercolor. It's what attracts most people to the medium, both painters and patrons.
On rare occasions however, the judicious use of white paint can be the perfect accent to a watercolor painting. It can clarify branches in the midst of a thick dark mass of trees, create window mullions on a deep blue pane of glass, suggest subtle complexity in the background of a city scene and provide the interesting graphic accent of signage on a street corner.
The key is restraint. Be delicate with your brush. I use a rigger or a swordliner (a long, skinny brush with a good tip). I wait until I have completed the painting and done all I can without white paint, and then I add the little spots and accents. I like to continue a dark painted stroke (let's say, in a tree) with white paint, so the line goes from dark suddenly to white. It is unexpected, illogical and very exciting.
This is a scene of people waiting in line to eat at the Pantry in Los Angeles. It is a classic diner that is always open, and has been since 1924. You don't leave hungry. For this painting, I used Arches 140# hot press watercolor paper. I did a loose in sketch in pen and ink before applying bright colors of thick, juicy paint. I wanted this to have lots of color in the buildings, people and the cafe awnings so I really laid it on. I painted reflections in the foreground to create interest but in reality, it wasn't raining. I also left plenty of white space though out the piece: the door frames, window mullions, random little areas along the street, and the architectural detailing in the skyscrapers.
After all that, I came back in with white paint for a few little touches: the pantry sign, the street sign, a few window details and a little splattering here and there. Not much, but enough to give everything some sparkle and energy.