The Anand Karaj takes place in the gurdwara in the presence of the Guru Granth Sahib, and that shapes what everyone wears. Heads covered, shoes off, dress modest and properly formal. For the groom, the outfit is built around the dastar, tied as part of the preparation, with the sherwani or achkan chosen to work with it. Getting the turban and the sherwani to sit together is the first design decision, not something to sort out once the sherwani has already been cut.
The groom: sherwani, dastar and colour together
The dastar and the sherwani have to be decided as a pair, because that's how every photograph will show them. The most traditional pairing is an ivory or cream sherwani with a dastar that either matches or deliberately contrasts. A gold-on-ivory brocade sherwani takes a deep red or gold dastar comfortably. A plainer cream silk sherwani with an ivory dastar gives you something quieter, and for a lot of grooms something better.
Beyond ivory and cream, Sikh grooms also wear sherwanis in strong colours, deep red especially, which carries auspicious weight in Punjabi tradition, along with gold and the occasional jewel tone. Whatever you pick, hold the turban cloth against the sherwani cloth in daylight before you commit. Two colours that each look right alone can fight badly on camera.
Guests at the Anand Karaj: gurdwara etiquette and dress
Every guest covers their head in the gurdwara. Scarves are usually available at the entrance, but bringing your own is better manners. Shoes come off, so wear socks you'd be happy to be seen in. Dress modestly and formally; bare arms and short garments aren't appropriate inside. You'll also be sitting on the floor for a while, which is worth remembering when you pick your trousers.
Indian formal dress suits the occasion well: a fine kurta-churidar, a bandhgala, or a good suit. Keep the register dignified rather than loud, because the gurdwara isn't the place to be the brightest thing in the room. Rich but restrained is the tone. Men generally avoid white, which carries associations with mourning in Punjabi tradition.