ONE accessory more than any other brings mystery to the human face . . . sunglasses. Immediately one conjures up icons: Greta Garbo scurrying for cover from the paparazzi, Sean Connery as James Bond undressing a bathing beauty from behind those dark lens, then dipping them lower down his nose for a second inspection. Then there's Hongkong's Cecil Chao who gets the thumbs up for being cool, parading in his dark glasses day and night. After all, what makes a 24-year-old model wearing nothing but body-hugging, rhinestone-studded La Perla dresses not only fall for a 55-year-old man but marry him as well? If truth be known, Cecil Chao has sex appeal, and why? Because, to use that '70s word which has yet to find a replacement, he looks ''cool'' because he wears shades. So if you want to look cool in the shade, take a tip from Cecil, just hide those eyes behind a dark screen. Sunglasses give a person mystery, drawing the curtains on the windows of our moods. Better still they hide hangovers, bloodshot veins, dark bags, crow's feet and bulging eyeballs. They provide a hiding place from the world. Today, the most devoted fans of sunglasses are the ones at international designer collections. Rain or shine, they will roll into town with their eyes hidden by shades. One wonders how they see the clothes. What they do know, is that sunglasses are a big fashion story for autumn-winter 1993-94. A wide range of eye wear was shown at collections in sophisticated yet sexy styles with the most popular inspired by the gangster movies of the '30s. But one of the most exciting developments on the sunglasses front comes from Swatch. The company's innovative clip system gives a new meaning to the phrase ''you've been framed''. It means you can change your look to suit the mood or clothing of the moment. Swatch Eyes consist of a slim-line basic frame on to which ''Beauty Mask'' clips of different designs can be interchanged. Once the wearer has a frame and clip, they can add to their accessory collection by choosing Beauty Masks to suit their moods and styles. The 1993-94 Spring/Summer Swatch Eyes range of Beauty Masks has four styles and five designs with 20 fashionable and colourful clips, some of which we introduced to Mr Chao. Swatch Eyes are manufactured under licence by leading German spectacles specialists, The Ferdinand Menrad Group. They don't only look good, but do your eyes a favour at the same time. The range is shatterproof and has four different levels of glare protection. Harmful ultra violet radiation is also screened out. The Swatch designers have sought to make Swatch Eyes interchangeable and inexpensive. Priced at $400 for a set (basic frame and a clip), Swatch expects fashion conscious Hongkong to swoop on the new range as additional clips are only $250.