I rung-up the overseas sweatshops and ...
... done!
Seriously though, there was this paper at the 2002 SIGGRAPH entitled
Transferring Color To Greyscale Images; Welsh, Ashikhmin, Mueller
that looks like it was made for colorizing movie dailies long after the fact. It described the research into a semi-automated process of transferring color from a source to a similar, even though B&W, destination.
The technically minded will find the available over-10-years-old .PDF an interesting read. The rest of us can just watch the pretty demo videos. 