I would go for B&W, high ISO, and concentrate on doing things that the official photographer cannot concentrate upon. He does all the formals, the groups, etc. and you do the candids, the "photojournalist" style.

If you are working indoors, colour would add technical pressure because you would have to work with lower ISO, worry about colour balance, probably not using flash not to attract attention etc. Considering you are not in control of the situation (directing people, for instance, to move them in favourable light) I would just concentrate in delivering some nice pictures that the couple wouldn't otherwise have had rather than testing how good film can be in a wedding.

The Leica suggestion, if you are "proficient" with it when focusing indoor, is a very nice advice indeed. High ISO, B&W, Leica, if the shots are good that makes a very nice manifesto for film photography in any case.

For outdoors pictures you can and should use colour negative but, unless all the party - dinner takes place outside, I would say that your most precious opportunity to "add value" to the official photographer's work will be indoors and, in that case, I would go with B&W 3200 ISO and concentrate on candid portraits of people.