Textures may be used for repeating patterns, or where cutting up the model to add colors may cause problems.
Complex text or symbols can use a transparent texture, and of course posters and pictures are alright.
For more info on textures see the 2D Textures Page.
For simpler things consider just using polygons.
These simple decorations are floating just above the object’s surface. This also includes the “fake” grates on the lockers. They’re too small and the camera too far for the player to ever notice.
