You can have only one residence/domicile at any given time, and your college address may or may not be your voting residence. It depends on whether you established residence/domicile in your college town.
Living in a location while you are temporarily going to school doesn't necessarily make that your residence/domicile for voting purposes, even if you lived there for several years. However, if as a student you registered to vote in the college town, got a driver's license from that state, rented an apartment there, and basically conducted your life as a resident would, then you established residence/domicile in the college town. If you did not do these things, then your address prior to moving to your college town (likely your parents' address) would be your voting residence.
Example One:
Mary graduates from high school in Texas and then attends Northwestern University in Evanston, Illinois, for four years. While she is a university student, she registers and votes in Illinois. Upon graduation, she joins the Peace Corps. She goes to Washington, D.C. for one month for training, medical examination, etc. and then is assigned to Malawi, a country in Africa. When she completes her Voter Registration/Absentee Ballot Request form in order to vote from overseas, her last U.S. residence address is where she last lived in Evanston, Illinois, and she will receive an Illinois absentee ballot. Note that she will not vote in Washington, D.C. because she was only there temporarily and never resided there.
Example Two:
Joe graduates from high school in Buffalo, New York, and then he attends Columbia University in New York City for four years. While he is a university student, he votes by absentee ballot, using his parents' address in Buffalo as his voting residence address. Upon graduation, he moves to France. When he completes his Voter Registration/Absentee Ballot Request form in order to vote from overseas, his last U.S. residence address is the address where he resided in Buffalo. Even if his parents move away from that address, or pass away while he is living overseas, he should still use that address as his voting residence address for as long as he lives overseas or until he moves back to the U.S.