Insulator disc

Insulator discs or optic discs were a feature of the Dalek eyestalk whose purpose was to protect the eye lens from deadly radiation. They varied in colour, size and number according to the different types of casing. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe, PROSE: Dalek Combat Training Manual)

History and variance
For a substantial portion of Dalek history, the insulator discs were coloured blue, including those of the silver Skaro City Daleks in the Thal-Dalek battle, (TV: The Daleks) the silver Dalek Earthforce including the saucer commander and Supreme Controller, (TV: The Dalek Invasion of Earth) the silver Daleks, their contemporary black Supreme Dalek, (TV: The Chase) and Emperor's Personal Guard, (TV: The Evil of the Daleks) the early grey Daleks from Davros's prototypes (TV: Genesis of the Daleks) to the Dalek-Movellan War, (TV: Destiny of the Daleks) the Gold Dalek, (TV: Day of the Daleks) and the Silver Daleks participating in the Exxilon Gambit. (TV: Death to the Daleks) The gold-and-black Dalek Supreme notably had black insulator discs. (TV: Planet of the Daleks)

The eyestalks of the Dalek War Machines in the Dalek City were adorned with nine blue insulator discs which increased in size as they met in the middle. (TV: The Daleks) The Time Lords observed that the later silver Daleks, as well as being fitted with solar slats, boasted improvements to the visual systems in the form of a recalibrated eyepiece lens with re-engineered optic discs. (PROSE: Dalek Combat Training Manual)

Following the Dalek-Movellan War, the grey Daleks and their Supreme Dalek had white insulator discs, (TV: Resurrection of the Dalek) as did the Necros Daleks created by Davros. (TV: Revelation of the Daleks) Davros's refined Imperial Daleks had gold insulator discs in line with their livery, whilst the now Renegade Daleks and their Supreme Dalek had white insulator discs. (TV: Remembrance of the Daleks)

Bronze Daleks and their contemporaries, from Imperial Guards to Black Daleks to Supreme Daleks, had white insulator discs. (TV: Dalek, The Parting of the Ways, Army of Ghosts, The Stolen Earth)

The original casings of the New Dalek Paradigm had black insulator discs. (TV: Victory of the Daleks)

The eyepiece of the reconstructed reconnaissance scout Dalek had no visible insulator discs, (TV: Resolution) nor did the Defence Drones which were derived from it. (TV: Revolution of the Daleks)

Behind the scenes

 * In Dr. Who and the Daleks, the Blue Dalek drones and the Red Dalek had blue insulator discs like the Daleks in the contemporary television series, whilst the Black Dalek had gold insulator discs. In Daleks' Invasion Earth 2150 A.D., all Daleks had blue insulator discs.