It didn't but when the episodes were found they were in black and white
The Mind Of Evil originally aired in full colour. After transmission all 6 master videotapes were wiped by the BBC for reuse. This was normal BBC procedure at the time. The 6 episodes of The Mind Of Evil that do exist were black-and-white film copies made for sale to overseas countries that, at the time, did not have color TV broadcasts.
- Plans are for the episodes to be restored to color using the same techniques being applied to The Ambassadors of Death and other stories. This is why Mind of Evil has not yet received DVD release (and won't until sometime in 2013, making it one of the last classic era stories to be released on DVD).
- Depending on the copying technique used, the monochrome copy may retain the colour information. Even if it does, it's a major task to retrieve that information & restore the colour version. Most fans, though, would probably agree that it's worth waiting for.
- Straight out colorization can also be done, just like you see with old Three Stooges movies. It's less objectionable since it's simply an attempt to restore was originally existed, rather than guessing what colors might have been correct.
- Colourisation is not "an attempt to restore was originally existed". It's colour restoration, as with The Ambassadors of Death, The Mind of Evil &c, that is aimed at restoring what originally existed. Colourisation of old monochrome movies/recordings is adding something that was never there in the first place. It can sometimes be done convincingly but the colours are someone's idea of what might have been -- an invention, not a restoration. (By the way, the Three Stooges aren't well known in Britain, unlike, say, the Marx Brothers.)