Why does a red or yellow border appear around visual components during runtime.

A red border around a component indicates an invalid link to a variable. This way, a component indicates it is linked to a variable that cannot be found during runtime. The variable might belong to a device not connected to the current computer or it might have been deleted via the Device Manager. This indication is controlled by the "Mark components with invalid links to variables" option (Reliance Design | Project | Options | Miscellaneous | Display).

A yellow border around a component indicates an invalid value. This way, a component indicates it is linked to a variable whose value is not valid. There might have been an error in communication with the device that the variable belongs to. This indication is controlled by the "Mark components linked to variables with invalid values" option (Reliance Design | Project | Options | Miscellaneous | Display).