Unfortunately during the Crimson Days event on Destiny, Ghost Shells had an unusually low drop rate. To remedy this, developer Bungie will be providing anyone that participated in the PvP event a max-level (320 Light) Ghost Shell.
Once Crimson Days concludes this week, the Ghost Shells will be delivered to players through the Postmaster. To qualify for the reward, you will have needed to partake in a minimum of seven matches.
There is no way to know whether you will receive the Chocolate or Crimson ghost, as there is no option to choose which you receive. Only one Ghost Shell will be awarded per account and should arrive at the Postmaster by February 23.
The next Destiny event is due in spring and promises PvE content and a new Light level cap, which will effectively make these Ghost Shells useless. Enjoy!