Screening colonoscopies will be performed on asymptomatic patients when they have attained the age for screening as a routine purpose. So you are right with using V76.51 (Special screening for malignant neoplasm, colon) as the primary diagnosis code when the first time the patient presents to your gastroenterologist and he decides to do a screening.
When your gastroenterologist records the patient's history and finds a family history of malignant neoplasm of the gastrointestinal tract or a personal history of malignant neoplasm of the large intestine, these findings should be recorded as secondary codes. So you will use V10.05 (Personal history of malignant neoplasm, large intestine), V12.72 (Personal history of colonic polyps) or V16.0 (Family history of malignant neoplasm, gastrointestinal tract) if your gastroenterologist records pertinent history in the documentation.