Colin and Dennis are Muggle-born, their father is a Muggle milk man. Colin was very excited to discover he was a wizard, and decided to take a lot of pictures to send home to his father, and Harry Potter was at the top of his list of things to photograph. Colin followed Harry around and constantly annoyed him. Colin is in the same year as Ginny Weasley. Dennis is Colin's two year younger brother, and is(like his older brother) in Gryffindor. Both of them joined Dumbledore's Army, but neither were involved in the battle in the Department of Mysteries.
Hugh Mitchell is the actor who played Colin in Harry Potter and the Chamber of Secrets(2002).