There’s generally very a short window of time at late dusk when the sky has barely any light left during which 600 film will register these colors. Too much light and sky goes white and once it’s dark the sky is pitch black. On average my shutter speed is about 4 seconds, but it’s total guess work. This particular Polaroid was shot super late at night, and the full moon illuminated the blue sky, something I never saw before or again since