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.
Portra 400 - a very flexible film.
Portra 800 is still available as well.
And I would be tempted to shoot the Black and White on Ilford XP-2 and have my lab make proofs of the whole shebang.
Kodak's Portra 800 is a very flexible color film that will handle black and white wedding attire and provide great skin tones. It is fast enough for indoors, yet works outdoors as well. And it can be pushed all the way to 3200 though you will certainly lose some shadow detail. If you can't find that then use Portra 400. It does not push as well but it can do ISO 800 in a pinch. Portra 160 is a wonderful wedding film as well but is best used outdoors only.
Yes, Portra 800 and Portra 400 are the way to go. If you have a camera with good TTL flash, and the matching flash, that will help at times also. Know how to use bounce, diffusers, and all that, on the flash.
Well, whatever you end up shooting, test it BEFORE the wedding, you do not want to screw up because you haven't shot the film before. Get to know how it responds to different things such as underexposure and overexposure. It will definitely be to your advantage. Also, stick with what you know and comfortable with, don't change just because you think it might look slightly better; it's not worth the risk.
I learned this the hard way... :whistling: