"The Queen tends to always wear one color because she feels that if people are queuing for many hours just to see her, the courtesy she should show them is make it so they can see her," Hughes told "Good Morning America." "Those in the audience will be able to pick her out and say they saw the queen."
That is also the reason you likely will not see Queen Elizabeth wear a green coat and hat to a garden party, for example, where she would run the risk of blending in.
She always picks contrasting colors and ones that are diplomatically and culturally appropriate to the event, according to Hughes, whose book will be available in the U.S. in October.
"So much thought goes into it and nothing is an accident," she said. "If anyone thinks they’re overthinking the queen's fashion, they’re not, because they would never be able to think about more than the queen and Angela Kelly.
Kelly is Queen Elizabeth's personal fashion expert. She started working for the royal family after the queen met her at the home of the British ambassador in Germany, where Kelly worked as a housekeeper at the time.
Kelly worked her way up the ranks and now even designs many of the queen's clothes and hats, according to Hughes. The pair work together, relying on no outside stylists, and pore over every detail of each event Queen Elizabeth attends.
"Everything is considered," said Hughes. "All the clothes are weighted down with metal weights, fabrics are packed up for several weeks to make sure they don’t wrinkle and new shoes are worn endlessly around the palace to break them in."
Speaking of shoes, those and her purse are the only accessories where Queen Elizabeth breaks away from her monochromatic look. She always wears patent block heel shoes and carries a handbag, both in white or black, depending on the season.
Elizabeth Holmes, a California-based journalist who chronicles the royal family's fashion at So Many Thoughts, calls Queen Elizabeth's fashion consistency "admirable."
"She knows what works for her," Holmes said. "There is something so refreshing in her color choices, especially as so much of fashion has veered towards neutral shades. The queen’s style is very much her own, and she wears it well."
When the royal family makes appearances together, like they will at Saturday's annual Trooping the Colour, their outfits are planned around the queen's wardrobe choice, according to Hughes.
"Whatever the queen is going to wear, everyone works around that, not because she rules with an iron fist but because the monarch has final say," she said. "She wouldn’t approve their outfits but they would know what the queen will wear."
With those details in mind, here are some of Queen Elizabeth's most colorful looks over the years.