Captain Indigo is an agent of Spectrum, who later was killed and reconstructed by the Mysterons in the original series, while in the other series, he is still active.
Original Series
Not much is known of Indigo's past except that he was probably one of the men chosen to a agent of Spectrum with the code name Indigo.
He is only seen in the episode "Spectrum Strikes Back" disguised as a lodge waiter while helping out his organisation in the development of two new Anti-Mysteron weapons.
After being asked to bring down one of the inventions, a C38 detector, his former fellow officer and Mysteron Agent Captain Black suddenly appears and shoots him. He is killed instantly and the Mysterons use their powers of retro-metabilism to make an exact likeness of him in their new scheme of destroying the information Spectrum knows about them.
Later the Indigo likeness comes down with the new detector and watches as Dr. Giadello gives a demonstration of the device which is shown to tell the difference between the x rays of a earthman and a Mysteron reconstruction.
When the device is suddenly used on him, he panics and quickly goes back upstairs. They examine the results and find Indigo's picture instead of an x ray which Indigo had known all along before his departure.
Indigo meanwhile sets the lift mechanism to crush everyone inside and takes the control panel key with him. He drives away, but Spectrum officer Captain Scarlet follows him in hot pursuit.
With the help of a mountain ranger, Scarlet manages to crash Indigo's car and in an insuing gun battle, attacks him with another of Dr. Giadello's inventions called the Mysteron Gun which fires a deadly barrage of electricity that kills Indigo permanently. Scarlet and the ranger retrieve the key from his corpse and Scarlet drives back to the lodge rescuing everyone in the nick of time.