In a recent interview with Hello!, the British artist opened up about the thought process behind the royal's heavily-referenced aesthetic.
"She always wanted to look gorgeous of course but there wasn't a signature make-up look. She never wore anything too flashy and was always very stylish," Greenwell explained.
"When I did her make-up, she only wanted touches that brought out and enhanced her beauty," she continued. "You would never do something so bold on someone of her beauty and calibre. You just want to enhance them to make them look as best as possible whilst looking herself."
With a series of high-profile engagements and red carpet photo calls on her regal schedule, there was undeniable pressure to present a photo-ready image to the world.
Sticking to what the duo knew best, there was one beauty item in particular that Princess Diana would never have worn - false eyelashes.
"You would never have tried something so bold such as false lashes," Greenwell revealed. "Why would you risk making her feel bad by doing something she wouldn't like?"
But Diana isn't the only royal to have delved into her make-up bag, as the artist-to-the-stars was also behind the Duchess of Sussex's fresh-faced appearance on her now-famous Vanity Fair 2017 cover.
Opening up about the shoot, Greenwell revealed: "She is lovely and I'm totally in awe of her. She's clearly made Harry so happy and she is divine in personality. It was a lovely time to meet her just before her engagement."
In terms of the beauty mood board, she added that Meghan wanted something "very simple and classic" as photographer Peter Lindbergh preferred shooting natural skin.
When the photographer - who also shot the September 2019 issue of British Vogue, which Meghan guest-edited - sadly passed away in September 2019, the Duchess of Sussex took to Instagram to pay tribute to the industry heavyweight.
Alongside a never-before-seen photograph of the pair seemingly taken during the Vanity Fair cover shoot, the caption reads: "The Duchess of Sussex had worked with Peter in the past and personally chose him to shoot the 15 women on the cover for the September issue of British Vogue, which she guest-edited."
"There is no other photographer she considered to bring this meaningful project to life. 'Forces for Change' was the one of the esteemed photographer's final published projects. He will be deeply missed."