When was the right time to visit Lhasa where we can see tree leave in yellow or red?

Generally speaking, the leaves will turn to red in October. In addition, foreigners are not allowed to enter Tibet alone. You need to join in a tour agency and apply for the Tibet Travel Permit. As I know, TravelChinaGuide is a reliable travel agency. I've booked a tour from them. The contact information is traveltravelchinaguide.com. Hope helpful!