When designing a sign, the principle of contrast is important to note. Significant differences between sizes and dark/light colors matter when you want viewers to see your sign. In this instance, reflective material has been used to create a handsome way finding sign (center right of photo). But when the sun goes down and lights from passing cars and adjacent buildings glow, each letter reflects differently. This creates what is seen in the photo, unpredictable contrast between the letters and the background.
Day shot of signage, center right. Other images photographed at night.