The Time Lords identified a "superior" white insulating layer as a distinguishing feature of the casings of the Necros Daleks, which were created by Davros in opposition to the Supreme Dalek's grey Daleks as a prototype for what would become the Imperial Daleks. (PROSE: Dalek Combat Training Manual [+]Loading...["Dalek Combat Training Manual (reference book)"])
The Necros Daleks. (TV: Revelation of the Daleks [+]Loading...["Revelation of the Daleks (TV story)"]