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Harrold notes that more neutral colors are also required for nail polish and makeup, and calls dark nail polish an "absolute no-no."
"It is the more natural look, it's not anything too bright," he says. "Same with nail varnish and makeup. It's always very natural, neutral, skin tone colors… you don't wear dark or black or gothic."
"They tend to opt to wear more sort of neutral colors, just not to draw attention to themselves, really," he adds.
Black is also fairly off-limits when it comes to clothing too. Black is the color of royal mourning, according to Harrold, so you typically won't see members of the royal family wearing it.
"Diana, Princess of Wales, on a couple of occasions, famously wore black," Harrold says. "So it's not written in stone that they can't, but it tends to be something they wear only when a member of the royal family or someone close to them has passed."
Staying social media-free
As an American actress, Markle's Instagram account boasted bikini shots and was filled with photos of meals, dogs and friends. And the lifestyle blog she founded, The Tig, shared her thoughts on everything from fashion to her favorite vacation reads.
But as a royal, you won't see her posting to social media anymore.
Markle shut down her social media accounts in January and folded her blog before the royal nuptials. Instead, updates on her life are now shared via official royal social media accounts.
"She doesn't need her blogs or Twitters or any social media, because what she says and does can be presented through the organization that she's a part of," he explains.
"The simple reason is, they don't want them to put something that they shouldn't, and members of the royal family…they don't get involved in politics or gossip or anything," Harrold says. "It's safer to have it all run by the palace."
Steering clear of selfies
If you happen to see Markle out and about and hope to snap a selfie with the Duchess of Sussex, don't hold your breath. Selfies are typically out of the question.
Harrold says the reason they don't do selfies is because the royal family doesn't position themselves as celebrities.
"They're not celebrities, same with autographs. Royals don't sign autographs," Harrold says.
"That's the difficult part for Meghan. She is, or was, a celebrity who's now a member of the royal family, so it very much changes for her," he adds.
But she will have her own voice
Royal etiquette, Harrold says, dates all the way back to the 14th century, but the Brits have also taken traditions from around the world and made them their own. The reason for such rules is rooted in setting expectations when it comes to behavior between the royals and the public.
"The reason that they're there, especially when it comes to royal etiquette, is so that people know how to behave around members of the royal family so they don't feel uncomfortable, and also so the royal family knows how to behave," Harrold explains.
"For example, now when Meghan meets a member of the public, she knows that she has to offer her hand first."
Markle will also be lauded as a role model for women; it's already been said that she will be a "beacon" and "someone that people look towards." On Thursday, Markle was named one of the top 25 most influential women in Britain, according to UK Vogue. That means Markle must hold certain standards.
And while the monarchy might put an emphasis on etiquette, Markle will likely keep her individuality.
"It doesn't mean to say that she won't maybe, perhaps come up with one or two of her own ways of doing things," Harrold says.
He points to the example of hugging the public. Members of the royal family traditionally do not hug, but Princess Diana would hug, and would get down to the level of children to give them a squeeze. Now younger members of the royal family tend to hug too.
