a lot depends upon what you have to begin with and what you are hoping to achieve.
If you mean taking an old black and white movie and adding some "colour" to it to make it less drab, then use the duotone filter and perhaps give it a sepia look.
You can download Stefan Burgers FREE FX Bench
http://www.burgers-transition-site.de/d ... index.html
If you want to do what some of the Holywood Studios have been doing and make a colour picture out of a black and white one I think you might need something a bit more expensive.
For Still photographs you may be interested in looking at what they do on this site
http://www.colourize.co.nz/
(Now THAT is painstaking.)
Steve J