For a couple of years a number of members of the Doctor Who Restoration Team have been investigating ways of restoring colour from black and white telerecordings (black and white film copies taken of colour TV shows for overseas sales).
Basically when the b/w copies were taken, some of the chroma (colour) information wasn't properly filtered out and becomes evident as a fine pattern of dots on the recording. The theory went that if you could decode these dots you could then restore the colour, frame by frame. Easier said than done.....
It's not been an easy task but finally this weekend the BBC will be airing an episode of Dads Army in colour that, although originally made in colour, has only existed in black and white for the past 35 years.
Newsnight feature here that explains all:
http://news.bbc.co.uk/1/hi/programmes/newsnight/7778770.stmThe restored episode airs at 8.35pm this evening (Saturday) on BBC 2.
Hats off to the guys behind it. Another one of their technologies will be on display as well when the episode is aired. About 10 years ago they invented a way to restore a "video" look to film recordings. So you'll be watching a black and white film recording restored to full colour interlaced video. And judging by the advancements they've made with Vidfire (the film to video technique) over the past few years, the future for the re-colouring of episodes of all sorts of things looks rosey.
Full details better than I could ever explain from The Guardian earlier in the year which explains both techniques and why they are so wonderful:
http://www.guardian.co.uk/technology/2008/mar/06/research.bbc