Protective coating

The Time Lords identified the protective coatings of Dalek casings, bonded to the Dalekanium shell. They were aware that Davros' original Dalek prototype had a toughened grey coating. In the "early period" of Dalek history, Daleks were constructed from raw Dalekanium, appearing in a silver form which was later adapted with new protective coatings. Davros' prototype design was replicated for later design units with variant coatings resulting in different shades of grey. When Davros created a new generation of Daleks, he invented a superior white insulating layer. The bronze Daleks, believed by the Time Lords to have been created for the Time War were constructed from a new Dalekanium alloy. (PROSE: Dalek Combat Training Manual)