I plan to travel to Jiuzhaigou on 31/10/11~2/11/11

Yes, still can see Autumn view at that time, but not the best. It has been almost in winter. Less crowded. The most crowded time would be early October. Huanglong will be open during your visit. Probably to rain in those areas. So, you should check the weather report before visiting and bring umbrella. The temperature in Jiuzhaigou at that time would be 5-15 degrees and perhaps colder if it rains. Need to bring down coat. Wear two layers of trousers. In fact, it may snow at that time.