gaw
The midrash says that the first shabbos of Adom Harishon was kulo yom it did not have a night. This is the source not saying good night as shabbos is a day of kulo ohr. It is definetly a minhag not to say good day good morning or good night. I agree with you that the idea that I derived from vedaber dovor is my own idea and I still think it stand true that the torah teaches us to talk differently on shabbos saying good morning or good night I believe takes away from kedushas shabbos.By the way a minhag is stronger than halacha.