From Debrett’s, aka the absolute oracle of the etiquette market:
Is it now acceptable for guests to wear white?
In short, no. Despite a rise in guests considering cream jumpsuits and definitely-not-bridal white shift dresses acceptable attire, Debrett's say that wearing white to someone else’s wedding is still a no-go. “It is best to avoid white or cream and black,” the Handbook states. “Overly bright colours and patterns should be avoided as they may dominate photographs, and care should be taken not to overshadow the bride. It is not necessary for all the components of an outfit to match.”