The Ambassadors of Death
Wiki says it best.
Although the entire story was made on colour videotape, only the first episode is retained in this format. In fact, it is the earliest episode that survives in the series' original videotaped format, either in colour or black and white. The remaining six episodes survive only as black-and-white film recordings and poor-quality domestic colour recordings made from a US transmission in the 1970s. This recording is severely affected by a rainbow-coloured pattern of interference that at times overtakes the entire picture
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