This is a painting all about the small piece of light or white paper that moves along through the center of interest, it slips in and out of the buildings, weeds and trees. White shapes should have a variety of edges, soft, hard and texture and have movement or direction. Try not to make them just a flat boring stripe across the middle of your painting. Entertain the viewer inside of the shape, putting texture, thoughtful details, and interesting color accents. It is a hard thing to learn to do but well worth the effort in transparent watercolor. As always, use a really expressive brush and try to establish the form of the objects with the type of brush marks used. This is just as important in the very first marks of the painting as in the last details. I actually believe you would be better off worrying more about the brush work, and value and edge control than using all of the bright expressive colors available to us today.