It was probably just a colour cycling thing - all the computer colours are morphed into white.

All of the series' of Knightmare are still held on film, including the pilot from 1986. BBC stopped getting rid of archive material in about 1978 as Neil said, when they realised they were paying 10x more to other companies for material they wnated to buy back. (eg dr who, most of the black and white eps were wiped in 1972-1974). Poor BBC lost a LOT of money due to this, and are still doing so now.  They have learned their lesson....