What Causes Greenish Discharge From The Surgical Wound?

Colon surgery 1-8-15....all well with that. Questions about wound - had a pocket, had to reopen wound and we are packing it 2x per day. Have been back & forth to surgeon. Thursday, March 12 went to ER w/greenish discharge, pain, low grade fever. Blood tests showed no infection, good white blood cell count. CT scan showed no more pockets in abdomen, looked better than CT scan 1 week earlier. Nurse removed packing, some discharge (greenish, gray) and said wound looked pink & pretty . No red around area. Today discharge on packing is BRIGHT Green.