If you think the nail polish rules are strict at private schools, then think again.
The royal women are known to follow a pretty strict set of grooming rules most of the time.
One of the most noticeable rules is that royals are not allowed to wear bright nail polish colors.

Apparently, Queen Elizabeth finds them to be “vulgar” (viaInsider).
The Queen’s favorite shade is Ballet Slippers.
This isn’t exactly the only exception to the rule either.

Apparently, the natural-looking nail polishes and other royal protocols must only be followed for state occasions.
Royals around the world have worn it."