post the picture, & spell out exactly what you need to do... like for example, apply 6 different colors in several places each,
...or is it simply changing from greyscale to.. "tanscale" for example... shades of tan, lighter & darker according to the gradiations of the original? This would be much easier...
..then what will be done with it? Like for example edgeprinting a "tanscale" print... in this case you would do almost nothing to the file, just set it up properly, use tan foil... done! It may be many hours of work as has been indicated above... but if we got a better look at the project, it could also turn out to be extremely easy.
If you show us the shell it may turn out that a similar, perhaps superior, version of that image may be available in a popular clipart collection someone here might be able to direct you to.