Could be balmy, could be too cold. Be prepared for both. Don't expect heaps of rain. That's a fab time of the year except for the smog which will envelop the city then. If you hate smog, come in one of the summer months but if you do, be prepared for the possibility of typhoons disrupting your stay. Hong Kong looks great under the summer sun because everything's green and lush, the sky is a brilliant blue too. If harbour views from the Peak matter a lot to you, come in summer. If being comfortable while you walk around matters more, come in winter. Good trip.