Actually, if the Colonel's father should somehow obtain a marquessate or dukedom, the Colonel would be style Col. Lord Firstname Fitzwilliam. A Lord Lastname always has a title of Lastname, whether it be his own or a courtesy title from his father.
You may observe this for example in Dorothy L. Sayer's Lord Peter Wimsey novels: Gerald, Peter and Mary are the three children of, I believe, the 5th Duke of Denver. They all have the family name of Wimsey. When the 5th Duke dies, Gerald Wimsey becomes the 6th Duke. Peter and Mary, as a younger son and daughter of a duke, are called Lord Peter Wimsey and Lady Mary Wimsey. (Peter, incidentally, serves as a Major during the Great War, but is called simply Major Wimsey; I assume for reasons of practicality since Major Lord Peter Wimsey doesn't really lend itself to shouting in the trenches.) Gerald's son, in turn, takes the courtesy title of Viscount St. George and is thus Gerald Wimsey, the Viscount St. George; his sister is Lady Winifred. When Lord Peter marries, his wife, being untitled, becomes Lady Peter Wimsey and takes the family name of Wimsey. When Lady Mary marries, she remains Lady Mary, but takes her husband's family name. (I omitted the names of Peter's and Mary's spouses in case anyone hasn't read the series yet and doesn't want to be spoilered.)
Clear as mud? It took me years to remember all the conventions about titles and names and courtesies and I still mix stuff up occasionally.
You may observe this for example in Dorothy L. Sayer's Lord Peter Wimsey novels: Gerald, Peter and Mary are the three children of, I believe, the 5th Duke of Denver. They all have the family name of Wimsey. When the 5th Duke dies, Gerald Wimsey becomes the 6th Duke. Peter and Mary, as a younger son and daughter of a duke, are called Lord Peter Wimsey and Lady Mary Wimsey. (Peter, incidentally, serves as a Major during the Great War, but is called simply Major Wimsey; I assume for reasons of practicality since Major Lord Peter Wimsey doesn't really lend itself to shouting in the trenches.) Gerald's son, in turn, takes the courtesy title of Viscount St. George and is thus Gerald Wimsey, the Viscount St. George; his sister is Lady Winifred. When Lord Peter marries, his wife, being untitled, becomes Lady Peter Wimsey and takes the family name of Wimsey. When Lady Mary marries, she remains Lady Mary, but takes her husband's family name. (I omitted the names of Peter's and Mary's spouses in case anyone hasn't read the series yet and doesn't want to be spoilered.)
Clear as mud? It took me years to remember all the conventions about titles and names and courtesies and I still mix stuff up occasionally.