WEMBLEY'S assertion that the Hong Kong Stadium turf will ''look perfect'' for the Sevens will not be far off the mark - although nature will play little part in revitalising the grass. A good proportion of the grass died because of the covering placed over part of the pitch during the orgy of concerts at the Stadium. The resultant brown look for the Asian Cup Winners Cup final on Saturday had Sevens organisers shaking their heads. In order to live up to their promise that the playing surface will ''look perfect'' on Saturday when BBC cameras will be flashing pictures of the Sevens worldwide, Wembley is planning to spray the grass a verdant green.