You can turn off subscription auto-renew at any time through iTunes. See http://support.apple.com/kb/ht4098
Your subscription will be canceled after your current trial or subscription period has expired. You cannot cancel a current subscription period once it is active. When your subscription expires, you will no longer be able to open pictures you colored as a subscriber, but you will be able to access any free features available to non-subscribing users on first download of app.