Have you ever noticed the colored dart boards like hangings on the ceiling of metro stations? If you are a frequent commuter, then you must have seen this, inside the stations many times.
Generally, these disks are installed, where flocks of the pigeons are known to frequent. In order to make the tracks and station clean from pigeons’ dumping, the boards are installed on the ceilings, it scares a pigeon from sitting on the ceilings, hanging pipes, etc. This type of colored disk usually, scares these avian menaces, so they aren’t able to sit near these disks, which further ensures the cleanliness of the station and tracks.