When you're a member of the British royal family, there are certain aspects of your life that are, for the most part, out of your control. They're expected to behave according to strict traditional rules that have been around for decades.

For instance, royal women are only permitted to wear natural nail polish colors when attending official events. This is why you never see Kate, the Duchess of Cambridge or Queen Elizabeth II wear bright-colored polish. In fact, the Queen has been wearing Essie's Ballet Slippers shade since 1989, and according to the company, "it's the only color Her Majesty would wear."

Other aspects of their appearance, such as hairstyle, are also under constant watch, which is probably why you never see a royal with an unnatural hair color. Perhaps, this is the reason why everyone goes bananas whenever Kate switches up her hairdo.

However, she's far from being the only royal who has opted for a drastic change. Her late mother-in-law, Princess Diana, also chopped off her locks back in 1990, and caused quite the stir when she debuted her rebellious new look.

The people's princess's former hairstylist, Sam McKnight, recently penned a book titled Hair by Sam McKnight, and in one of the chapters, he reveals the real story behind Diana's iconic short hair cut.

According to McKnight, the late princess got her hair cut in the moments after her 1990 Vogue cover photo shoot.

During the shoot, McKnight gave Diana a faux crop by tucking her medium length hair around her tiara. Little did the stylist know that the princess was falling in love with the shorter look.

"I made her hair look short in the tiara for the shoot and she decided she liked it," McKnight wrote.

McKnight continued, "As she was leaving, Diana asked what would I do to her hair if I had free rein.... I suggested cutting it short..."

Without hesitation, Diana agreed, and "we did it there and then," recalled McKnight. The pair took a series of photos afterwards, which McKnight included in his book.

Apparently, Diana was fearless when it came to trying new things, and according to her former makeup artist, Mary Greenwell, "she loved makeup and loved experimenting. It was just much more about really getting her to look absolutely beautiful all the time."

She, however, knew where to draw the line and made sure she still abided by the royal codes.

"She was someone who was in the public eye presenting herself all the time to the public," Greenwell said. "It isn't particularly appropriate to go on changing your look every five seconds. She did change her makeup a bit. It's just that it wasn't changed dramatically."

McKnight's story shows us a totally different side of Diana we did not have the chance to see very often.