Etiquette may help Kate Middleton decide which demure outfit is best for a royal wedding or a polo match.
But when the occasion is a charity roller disco - and you're the organiser - it's a difficult one to call.
Last night, the 26-year-old, who once worked at the fashion chain Jigsaw, decided luminous yellow hotpants and leg warmers fitted the bill.
And Prince William's girlfriend was in good company. The event in Vauxhall, South London, attracted hundreds of guests - many wearing equally garish outfits. Her sister Pippa, 23, also opted for pink legwarmers to brighten up her black dress.
And Richard Branson's 26-year-old daughter Holly, who was also helping to organise the Day-Glo Midnight Disco, wore a pink tutu over her bright orange ensemble.
Miss Middleton didn't let sombre black boots detract from her look for long, however. She soon took to the floor in bright pink roller boots.
Hundreds of supporters joined her in leggings and sweatbands as she aimed to raise £100,000 for Tom's Ward at the Children's Hospital in Oxford, and the organisation the Place2Be - which provides counselling to youngsters.
Tom's Ward is named in memory of Miss Middleton's friend Thomas Waley-Cohen, who died from bone cancer in 2004 at the age of 20. Miss Middleton went to boarding school with Mr Waley-Cohen and has been supporting his family's appeal behind the scenes.
Miss Middleton is said to be anxious to prove she is not sitting round 'waiting for William to click his fingers', as senior Royal sources have recently claimed.
Earlier this month her friends hit back, insisting she does have a proper job working for her parents' party accessory company.
She is responsible for 'compiling and editing catalogues' 9-5, five days a week, from the Middleton family home in Bucklebury, Berkshire.
A a close friend told Hello! magazine the reports were 'utter nonsense'. 'She is a young woman who wants to keep active and that is why she is annoyed at the reports saying she doesn't work,' the friend said.
Despite her friends' insistence, Miss Middleton has had only one job since graduating from St Andrews University in 2004 with a 2:1 in history of art. She joined the Jigsaw fashion chain as a buyer but quit the four-day-a-week job after less than a year. Plans to work in an art gallery were also put on hold.