I generally feel comfortable with using my professors first name. However, when I hear a student use doctor when referring to the professor, I typically call them doctor after that since I feel it is safer to in that case. I have noticed that most of the time, it is considered acceptable to use your professors first name in college. I feel like part of the college culture and how it differs from high school culture is that the professors treat you more equally to them as opposed to acting as an authority over you, and I feel like part of that is letting you use their first name. I have even had professors explicitly tell me and the class that it is okay to use their first name.