And ever since, Meghan has adhered to the rule - except for Thursday's visit to Marlborough House for the Commonwealth Youth Reception.
Meghan stunned in a summery yellow dress - designed by Lady Gaga's United States stylist Brandon Maxwell ≠ as a smart Harry looked dapper in a suit and tie.
To those who have heard Meghan speak recently, especially since her marriage to Prince Harry, the San Fernando Valley-born former TV actress sounds like she's acquiring certain vocal characteristics of a British accent.
And it appears that the Duchess of Sussex, 36, is taking tips from the Queen - who often wears colourful outfits to ensures she stands out in a crowd.
The fitted mid-length shift is believed to be a nod to her home in California - though the United States is not a member of the Commonwealth - and is by American designer Brandon Maxwell, who used to be Lady Gaga's stylist.
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Since all three were absent from this reception, Meghan chose to wear a more vivid shade than her usual pastels and whites.
Along with her canary yellow coat and hat at Ascot, the Queen also wore a bright green coat and hat during her joint appearance with Meghan in Cheshire last month.
While talking about what the Commonwealth youth might learn from their new ambassador The Duke of Sussex, and his wife, the Baroness said: "I think one of the things they will see is unity because both of them have a real interest in people". As you'll hear in the video below, Meghan's accent slips a touch into Dowager Countess of Grantham territory on a few occasions, particularly in the phrase: "We've all (awwwhhlll) had a great day".
In a speech at the event, Harry said: "Meghan and I are incredibly excited to meet so many of you representing the future of the Commonwealth".
At every official royal event Meghan has attended since marrying Harry (excluding Celia McCorquodale's wedding which was a private event), other senior royals have been present. "They were just laughing because they were joking about how they were exhausted and finally it was over".