When I use the "print_pl" command in simplewallet to see the peer list, the first attribute of each line is either "white" or "gray". What is the meaning of it?
White peers are online and reachable and grey peers are offline.
The longer period of time it has been since you last used your daemon, the more grey peers you are likely to see.
White peers that fail to upgrade during a hard fork are (eventually) dropped from the network and become grey. People move and change VPS hosting providers, so peer status will always change over time.