Modesty Is The Best Policy
Not only are there rules about wedding dresses, but in general women in the royal family have a strict dress code. Ladies must wear modest clothing, and bare legs are a no-no, so instead they were tights. Also, you will rarely see a royal with color nails, only neutral tones.
Stay Quiet
For centuries long the Royal family has been seen as figureheads of the country and not political decision-makers. Thus, the members of the royal family never express their political opinions or get involved in state affairs.