(click the image to swap between day and night)
With this project, I mainly wanted to work with completely erasing all sources of light and rebuilding them. After a simple sky replacement where I added a few clouds, stars, light pollution, and color/value adjustment, I started by tackling the neon signs following this tutorial. It follows a method of color overlays and glow through blending options, and explains it far better than I could, but here is a sample of how I did it.
For instance, take this area of neon signs.
Here is what it looks like color corrected, with no overlays, glow, or ambient light.
Now, here's how I achieved the effect. Firstly, I selected the signs in the ungraded image with their true, brightest colors. This already makes them begin to glow and stand out, as if lit from within, but they are only lit with sun.
Next, I added a glow to each sign since light does not exist in a vacuum, and an additional color overlay to some signs because their colors were not as bright as I had hoped.
I also realized at this point I had missed a sign. Whoops! But, this does not quite reflect the way light actually behaves in a night environment: it bounces, refracts, and reflects in a much less gradated way. So, following the aforementioned tutorial, I used the brush tool to mark out where light would fall, and then simply used blending modes to extract it from the shadows of the underlying image. With some refining, here is what I ended up with.
And voila! From day to night in no time. Here's a comparison between the simple grading and the version with lights in it.
For the remainder of the image, I used this process on other signs, street lamps, cars, and a few phones. I believe i was most successful in the signs; the trails from the headlights don't seem quite accurate, but I'm unsure of how else to get the desired effect. With more research, I may return to this. But, overall, it was a good exercise, and I feel like I am improving overall and learning! With some more practice and effort, my skills should be refined.