A. Bosch ceramic glow plugs are color-coded with a "white seal. ring" "arrow", as well as a chamfer on the top (-see the picture in enlarged form-).
A. NGK - ceramic glow plugs have a "sealing ring, white or (depending on the circumstances) with a silver coating" "arrow", but does not have a chamfer on the top.
B. Metal glow plugs have a "red seal. ring" "arrow" as a color mark.
Handling ceramic glow plugs
For ceramic glow plugs add. the following rules should be observed. Ceramic glow plugs are very sensitive due to the special properties of the material. They require a special approach during installation and removal. Transportation and storage may only be carried out in the original shipping containers or in separate packaging in air-cushioned film. Ceramic candles should be removed from their packaging only immediately before installation. Ceramic glow plugs are sensitive to impact and bending. Therefore, the fallen ceramic glow plugs (from a small height, about 2 cm) can no longer be used, even if there is no obvious damage (for example, hairline cracks). If there is any doubt about the perfect condition of the ceramic glow plugs, they should always be replaced. Damage or breakage of the ceramic glow plug pin will always result in engine damage. If the pin is damaged, all fragments must be removed from the combustion chamber before the next engine start. Otherwise, it will lead to engine damage in any case (piston scuffing). To check the compression pressure, select the "Self-diagnostic Systems" function in the "Guided Fault Finding" menu. Do not remove the glow plugs during the compression test. The engine control unit software is adapted to a specific type of glow plugs (ceramic or metal), therefore, use only candles of the appropriate type. Mixed installation of ceramic and metal glow plugs is not permitted.
[Information obtained from this resource: AudiManual.ru]