Practical lights are light sources that originate from within a scene.
The top photo shows a pub in Malta as it appears in daylight. The middle photo was taken at night, with the neon, fluorescent, and incandescent lights all interacting. Each has a distinctive color corona around it, and each influences the area nearby. Note the mixing of light on the awning at right.
The bottom image is a 5x7 inch watercolor painted at night from observation from across the street. Even though the neon was intensely chromatic and very high in value, I knew I would have to bleach it, flare it, and use the color corona to get across the brightness. I couldn’t go any brighter than the white of the paper.
The brightness and color of a practical light is more evident from its effect than from the appearance of the light source itself.
To do a plein-air nightscape, I had to have just the right illumination on my work, and fortunately, the park bench where I was sitting had some spilled light from a hotel behind me so I could more or less see what colors I was using.