The Departed (2006)

Answer: Colin is Damon, not a mixup. I agree that it is to protect her identity from someone who may enter the apartment.

Answer: My interpretation is to emphasize the divide between the two characters and show how the person one conveys on the surface could be vastly different from someone's true nature. Colin is a cop (arguably a good one who is granted his own team investigating organized crime). As a cop, society views him as a good, well-intentioned, selfless person who serves a greater purpose. Furthermore, we are shown that he is highly ambitious (pursuing a law degree and aspiring to a life in politics). All of this shows that on a societal level, he is a good fit or match for Madelyn. She is a doctor, therapist, university educated, decent person who would typically end up with someone in a similar social class and circle. This juxtaposes Billy, who despite the fact that he went to a prestigious and private private school, is extremely intelligent and we know from the beginning a good person, to the rest of society is simply a low-life murderer, criminal and thug and member of Costello's inner circle. This is not the kind of person that Madelyn nor society would pair her with. So, on the outside, the characters are written as such. But this small interaction with the childhood photo shows that surface interpretations are not an accurate representation of the true man. Colin isn't actually the right match on a more intimate level, and Billy, for all intents and purposes is more loving and compassionate to Madelyn's true self.