I have attended both two and four year colleges, and I've noticed a difference between the two institutions in regards to communication with instructors.
At my four-year College/University, I tended to use "professor" in both formal and written conversation when speaking to professors, unless the professor specifically stated otherwise. The department of Modern Languages tended to be different, as often the professor will request to be addressed in the language of instruction, such as "Madame LaRoche".
In Technical Colleges, the term professor is not as liberally used, as instructors often have the title of "teacher" or "instructor" instead of "professor". Many of my teachers in this setting have indicated a preference to be called by their first name for all communication both via email and in class.
As jimreed mentioned in a different response, if you don't know how to address the instructor, ask them. Not all professors (or colleges) are alike!