context: Harold Shand is the ruling kingpin of the London underworld and is trying to convince the American mafia to go into partnership with him, with plans to redevelop the London Docklands, including building a casino. However, his world is suddenly destabilised by a series of unexplained murders (including Harold's best friend, Colin) and exploding bombs from an unknown foe. He and his henchmen try to uncover his attackers' identity whilst simultaneously trying not to worry their American visitors, who Harold believes will not go into business with him if they think he's not in full control of his affairs. The situation worsens when a pub in which they are about to have dinner at is blown up and Harold's partner, Victoria, tells the Americans that they are under attack from an unknown enemy. She suspects that Harold's right hand man, Jeff, knows more than he has been letting on about the attacks, while Jeff simultaneously starts to fear that Harold might be onto him and tries to prevent people from talking through intimidation.
  After some detective work, Harold confronts Jeff, who confesses that he sent Harold's friend Colin on a job to Belfast to deliver money raised by Irish Navvies (construction workers) to the IRA. When Harold discovers Jeff accepted a job to deliver funds to the IRA, he becomes extremely angry and vows to destroy the IRA in London. Jeff advises against this and explains that the IRA aren't just annoyed about some of the money which had gone missing, but also wrongly hold Harold responsible for the murder of three IRA men. Under intense questioning, Jeff confesses that the IRA threatened to kill him and he was so scared that he "put the finger" on Harold, who loses his temper and kills him with a broken whisky bottle. Harold forces Councillor Harris to organise a meeting with the IRA, and appears to offer them Â£60,000 in return for a ceasefire but double-crosses them by having them shot and killed along with Harris.
  Having taken out the top men, Harold believes his problem is solved and travels to the Savoy Hotel to share the news with his Mafia partners. When he arrives, he finds the Americans are preparing to leave, having been spooked by the attacks from the IRA. Harold then delivers a short but powerful speech, saying he has lost all respect for the Mafia and accuses them of American arrogance. He finishes by telling them he has new German partners and that their services are no longer required. Harold leaves the hotel and gets picked up by his chauffeur and bodyguard, only to find the car has been commandeered by two IRA men who hold him at gunpoint. He sees Victoria being held in another car by more IRA men and, realising he is sure to be murdered, sits in silence, displaying a range of emotions.
question: What is the relationship between Harold and Colin?
answer: They are best friends