Slominer
I’m sure ANY color is ok…I’m only saying that at the chassanahs I’ve been to, where the bride wore a gown in anything other than white, it was a lighter pastel color. I’ve never seen purple or red for the kallah.
As to “minhag”, I’m using the term somewhat colloquially…its become a custom in both Jewish and non-Jewish weddings for a variety of reasons (no just the “sign of purity” etc.) not clear whether yidden or goyim started it first but I suspect its simply been woven into Western culture without regard to any considerations of halacha. Most kallahs fret about whether the gown is sufficiently tzinuyah, not whether the shade or white or slightly “off-white” satisfies some unrwitten minhag.