The paintings we did in class today relied heavily on the darks for their cohesiveness and content. We were free to apply the lights and middle values very casually. Most subjects need a little more deliberation in the early stages, but it still pays to look for opportunities to let the paint be in charge.
In this photo there are a few middle value shapes that require some care, like the domed basilica in the background, and a couple of lights, like the upward facing planes of the cars and the people's shoulders, that would be good to reserve, but it's still the darks that do most of the work of describing the narrative content. Can you imagine a mostly carefree treatment of the first layers, done in the faith that the darks will make sense of it in the end?
Each step of the way, this image keeps promising one more chance to make the illusion of light and substance convincing. If the yellow and the green ran together because you didn't wait for the first one to dry before adding the second, the red door frames would make it look solid. And if you made the frames all wobbly, the shadows would still make the light believable. And if your shadows were sloppy, the black doorways would sober the whole thing up. Inform yourself, and have faith!