Princess Diana's make-up artist reveals royal's beauty secrets

is considered one of the

of our age. In a rare interview to mark the 17th anniversary since her passing, one of the royal's make-up artists has opened up on some of

.

first began working with Diana when they teamed up in 1991 for

's December issue, which was shot by Patrick Demarchelier.

"She was

when we first met and had

," the A-list make-up artist told

. "Her marriage seemed solid and she was just

. It was a huge privileged to work with our then future queen." Mary revealed it was on their first shoot that she first

– "Blue eyes should never wear blue pencil or shadow" – adding, "From that moment on, she was open-minded about trying new looks and colours." She said, "

. I think as soon as we worked together

, and that is something that should never be underestimated. By anyone."

Mary also spoke about one of Diana's greatest beauty assets – her flawless skin. "

, which frankly at the end of the day also can come down to family genes," she said. "Diana was

, she would always make sure she took her make-up off at the end of the day and make sure she had a clean face before applying make-up. "…After I started working with her,

. She cut back on drinking so her skin was 100 per cent. Women today need to take care of themselves the same way. If you smoke, drink or do anything to excess, you will destroy your most important beauty asset – your skin." Mary also revealed her

. "We always started with a moisturiser, a foundation to suit her skin tone exactly and then used a concealer to lift the eye area. Mascara – which is a gift of all gifts – and lip colour to suit her mood that day. "I always taught her to make sure when applying mascara by herself that she covered the roots of the lashes too!"