First introduced during the second season, he has become a part of the team led by FBI Special Agent Don Eppes (Rob Morrow) and is usually partnered with David Sinclair.
This connection creates some tension when Colby is accused of being a spy for the Chinese which is confirmed in the season three finale episode, "The Janus List", with the news leaving the FBI team under Don's command shaken by the revelation.
This is proven to be true in the Season 4 episode, "Chinese Box" as he helps SWAT in the rescue operation to save David who is being held hostage in an elevator by a gunman.
At the end of Season 3, it was revealed that Colby had actually been spying for two years on his colleagues, working for the Chinese, as had his "old Army buddy" Dwayne Carter (Shawn Hatosy), whom he covered for and then sent to prison in "The Mole".
His fellow FBI colleagues were left stunned and shocked at the revelation, with his partner Sinclair taking the betrayal especially hard.
Any doubt of his status was finally removed when the agents rescued Colby from being killed by Chinese spies on a ship heading for international waters.
Dwayne Carter dies, saving Granger once more by taking out torture expert Mason Lancer (Val Kilmer).
When FBI Special Agent Roger Bloom (Henry Winkler), a 34-year veteran of the FBI's bank robbery division who lost his job because he stole 10 million dollars (US) to draw attention to a 2,000 dollar (US) theft at his sister's store, tries to solve a series of unsolved bank robberies, Colby relates his own experiences to Bloom.