The bride will be the one standing in the circle alongside the groom (or another bride, if this is a lesbian handfasting). Other than that, it can sometimes be hard to determine her from the other ladies present.
In Wicca, no book, no High Priest and Priestess, no vision from the Goddess Herself will demand that the bride wears a certain color. While guests at Christian weddings avoid white, and those at Sikh weddings avoid red, it takes some discreet questions to dodge clashing with a Wiccan bride.
I told in Fifteen Years of Wiccan Weddings about an impromptu handfasting. The bride was wearing what she'd had on all day. The only addition was a garland on her head, created from whatever was quickly found in the riverside hedgerow. She was no less handfast for that.
In all that time, I've seen every color of the rainbow. Each bride picks her favorite hue, or that of the most gorgeous gown or robe in the shop. I've seen plenty arrive in green, as that's the color of the natural world at the height of spring. Beltane or Summer Solstice are such common dates for handfastings!
Or they've arrived naked. That's an option too. It's precisely what happened in the Witch Wedding described in Stewart Farrar's What Witches Do. This ceremony was performed indoors, out of view from passers by. The Alexandrian coven adorned a couple of cloaks in the richest possible way, but they were worn over the naked forms of both the bride and groom.
It's way too cold to do that outdoors, at least in Britain. Traditional Wiccan bridal wear moves then to the next level. We ask ourselves what precisely is going on here for the bride.