What is the ruling on the exchange of juice drinks between the bride and groom at their wedding, their cutting and eating of the cake, and the bride wearing a white dress?

The default rule for such customs is permissibility, based on the principle: "The default state of things is permissibility." The evidence for this is Allah's saying: "It is He who created for you all of that which is on the earth." And the saying of the Prophet (peace be upon him): "What is lawful is what Allah has made lawful in His Book, and what is unlawful is what Allah has made unlawful in His Book; and what He has remained silent about, that is what He has pardoned." Therefore, they are not prohibited unless there is a reason that necessitates prohibition.