In a typical zombie movie, the zombies are mostly interchangeable, aside from the occasional standout who's funny in some way, like by wearing a thong or having a hammer stuck in its head. A single zombie is inconsequential, because it's easily outrun or dispatched. It's only when zombies are in a horde that they become a force to be reckoned with.
We never really think about the lifestyle of the individual zombie. How do you get your share of food? How do you become part of a horde? How do you stay safe? Colin takes a look at these questions through the story of the titular protagonist, who gets zombified in the first 10 minutes of the movie. As Colin (Alastair Curtin) wanders around the city, it seems like wherever he goes, the food easily gets away, or is already claimed by other zombies, or just can't be found. The first impression is that Colin might be a bit slow, both mentally and physically, even for a zombie, but then the realization comes that this is what life as a zombie would be like. You spend most of the time wandering around moaning, and once in a while you get lucky enough to find food that can't get away. If you're with other zombies, this food can be overcome. Otherwise, you're probably going to have to keep looking, if you survive.
There have been other movies that tried to tell with pathos the story of a single zombie, like I, Zombie: A Chronicle of Pain or Zombie Honeymoon, but in these stories the zombies were people who remembered who they were and understood what was happening to them (eventually). Colin manages to show pathos for someone who has gone full-zombie. He reminds one of a forlorn three-legged dog, awkward, hungry, and sad, and he definitely has lost most of his former self. Our sympathy for him is probably helped by the fact that we rarely get to see him eat anything.
The problem I had with the movie is that its ideas could have been shown in a lot less time. It doesn't take long to understand that the life of a zombie is mostly lonely and boring. To try to liven things up once in a while, writer Marc Price would mingle in scenes with living people (running the gamut from being eaten to turning the tables on the zombies and victimizing them instead), but we don't have any emotional investment in these living people, so we can't really bring ourselves to care whether they survive or not. There are some interesting scenes here and there, and one particular one that would have made a good ending, but for the most part the movie was pretty dull. It had the smack of a short film idea stretched to feature length, and if that was the case, Marc Price should have stuck with the short film.