Hi Sylvia,
It was from a photo. I started with broad watercolor washes and established the basic value and color pattern. I then layered thin acrylics. I changed colors (warm and cool) mixed with white each subsequent layer and painted it thin enough so that previous layers would show through here and there. I also used real fine detail hatching doing the same thing with blues, pinks, oranges, etc. In the end, I used broad washes of watercolor to tone things down if there was too much contrast. I think the key was getting the black background really dark and getting the sky the right value. Once they were in place it felt like a night scene.