The tradition of brides wearing white in royal wedding ceremonies is a longstanding one, but it doesn't necessarily have specific rules that apply universally to all royal families. The choice of a white wedding dress is more of a cultural and fashion tradition rather than a strict rule.
The popularity of white wedding dresses can be attributed to Queen Victoria of the United Kingdom, who wore a white gown when she married Prince Albert in 1840. This choice set a trend that many brides followed, not only in the UK but also in other parts of the world.
As for the belief that if someone else wears white at a wedding, their marriage will not be blessed or successful, it's more of a superstition or old wives' tale rather than a widely recognized rule. This belief is not specific to royal weddings but can be found in various cultures and societies.
In some cultures, it is considered improper for guests to wear white to a wedding because the color white is associated with purity and the spotlight is meant to be on the bride. Wearing white as a guest might be seen as trying to upstage the bride or being disrespectful.
However, in modern times, these customs have relaxed, and it's not uncommon to see guests wearing white or lighter shades at weddings, including royal weddings.
Ultimately, the success of a marriage is not determined by what guests wear at a wedding. The happiness and longevity of a marriage depend on the love, respect, communication, and commitment between the two individuals involved. Wedding attire and superstitions, while culturally interesting, do not hold any real power over the future of a marriage.