These are often used as "light-up" details in sets, such as street lamps, Arc Reactors in superhero themes, or glowing eyes for monsters.
If you are searching for "Dark 1x2" in relation to soccer or football, . Dark 1x2
In the world of brick building, a "Dark 1x2" is usually a specific 1x2 plate or tile made of phosphorescent plastic. These are often used as "light-up" details in