Colby Granger (portrayed by Dylan Bruno) is introduced as a rookie FBI agent in the TV show Numb3rs, who joined the team in the second season. David Sinclair is his usual partner. Granger idolizes the legendary sniper/tracker Agent Ian Edgerton. Granger had recently begun carrying the .45 ACP caliber Springfield Armory Custom TRP (Tactical Response Pistol) Professional Model, which indicates he is SWAT/HRT (Hostage Rescue Team) qualified, as only SWAT/HRT qualified FBI agents are authorized to carry that weapon on duty.

At the end of Season 3, it was revealed that Colby Granger was a spy for the Chinese, like "old Army buddy" Dwayne Carter (Shawn Hatosy) who he covered for and then sent to prison in "The Mole", and had been for at least two years. His fellow FBI team was shocked, with his partner David Sinclair taking the betrayal especially hard. Colby was immediately removed from the team and incarcerated. However, in the Season 4 premiere he escaped while being transported to another prison and it is revealed that he was actually only acting as a triple agent in order to help out the US counterintelligence. However, his supervisor and sole contact in counterintelligence, Michael Kirkland, was killed, making his status merely alleged. Any doubt of that was finally removed when the agents rescue Colby from being killed by enemy spies on a Chinese ship just short of international waters. Carter dies, saving Granger once more, by taking out torture expert Mason Lancer (Val Kilmer).

David was distant with Granger, which caused their team up to meet with disaster on a mission and Don to reprimand them. Since then, he had reconciled with Sinclair and was welcomed to stay with the team.