IT must be wonderful to be a pop star - I mean, in what other business can you enjoy the luxury of an extended adolescence? So it was last night at the Coliseum where Foreigner front man Lou Gramm - in regulation rock stage gear of jeans, sleeveless T-shirt and white trainers - bounced around the stage with the energy of a born-again teenager. This was rock music without the trimmings. Functional stage, minimum lighting and a scheduled start (well, 8.45 pm instead of 8.30 pm, but that is punctual by pop standards). With the minimum of introductions we were on with the business at hand - the belting vocals, soaring keyboards, thunderous drums and throbbing guitars of Long, Long Way From Home getting things off to a perfect start. Gramm's next belter was Double Vision and then, and only then, did we have the regulation ''Good evening, Hongkong . . .'' The audience, although small to middling, were an appreciative lot, among whom I spotted Canto-pop queen Sally Yeh, looking fetching in black. Also there was STAR-TV executive VP Arnie Tucker, whom I ran into prior to the concert on the concourse outside where he was promenading with his bubbly wife, Ilene (''It's pronounced 'Eileen', but her mother couldn't spell'', he helpfully offered). Mind you, Eileen (spelt 'Ilene'!) recovered smartly. As Arnie ordered her a cheese sandwich at the snack bar, she countered: ''Is this what you meant when you said you were taking me out to dinner?'' Back on stage, Gramm was introducing a number from Double Vision - ''an album which goes back about a dozen years''. A voice at the front whooped in delight - and I could have sworn it was Arnie Tucker.