The nikah is the marriage contract, read by a qazi in front of the couple, two witnesses and the families. What men wear follows from principles of modesty and respect rather than from a rulebook: clean, well made, dignified, without showing off. In India that usually lands on fine white or cream ethnic wear, though the exact shape of it changes a great deal between Tamil, Hyderabadi, Lucknowi and Kashmiri families. Know which tradition you're dressing for before you order anything.
What the groom wears at the nikah
Across most Indian Muslim communities the nikah groom wears white or off-white. The standard is a fine white cotton or silk kurta with a white churidar or salwar, and a white or cream sherwani over it. A nikah sherwani is deliberately less worked than a reception one: clean lines, good cloth, restrained detail. The formality comes from how well it's made, not from how much has been piled onto it.
The regional versions are worth knowing. Hyderabadi grooms often wear a sherwani with the embroidery associated with that tradition. In Lucknow, white-on-white chikankari on a fine cotton or mulmul kurta is traditional and, in good hands, exceptionally beautiful. In Tamil Muslim families the Pathani suit in fine white cotton is the common choice, and it suits Chennai weather better than almost anything else we make. Any of these can be cut here in the right cloth.
The walima: the celebration after the nikah
The walima is hosted by the groom's family after the nikah, and it's the celebration proper: bigger, louder, more food, more colour. The dress opens up with it. Grooms often change out of the white into something richer, an ivory sherwani with gold embroidery, a jewel-tone silk bandhgala, or a well-cut Jodhpuri suit.
For guests, the whole range of Indian formal dress is fair game, and festive is the entire point. Nobody at a walima is trying to be subtle. White tends to be left with the nikah, and richer colours carry the evening better in any case, particularly under hall lighting and in the group photographs everyone will be posing for.