Colon is a sergeant in the Ankh-Morpork City Watch, and appears to have been so for a long time. He was a corporal in the Watch at the time Samuel Vimes first joined, and subsequent to this spent some time in the army (the Duke of Quirm's Middleweight Infantry and then the Duke of Eorle's First Heavy Infantry), before returning to the Watch.
He is described on several occasions as "one of nature's sergeants". He is overweight, and prefers to avoid trouble and exertion. He is also rather unimaginative. When not on desk duty (a post he gets more often than other sergeants, due to being responsible for working out the rota), he generally "guards" bridges or large buildings against theft. His reasoning is that until someone attempts to steal the geographic feature in question, he can lean against it and stay out of danger (as well as the wind).
In The Fifth Elephant, Colon became the head of the Traffic Squad, which also included his best friend Nobby Nobbs. This role perfectly fit the above described qualities, especially as the Traffic Squad is "self financing" (i.e. they keep the fines). A brief promotion to acting-captain confirmed what everyone, himself included, had suspected, and he has returned to his previous rank. Once he put clamps on various buildings for "causing serious traffic congestion". Among the clamped were the Opera House, three other buildings, six fountains, three statues and a gibbet. He also clamped the Patrician's Palace for the same reason, but he let it slide because Vetinari parked it on business.
He is currently holding dual position of Custody Officer and Watch Liaison Officer; jobs so vague that no one is entirely sure what they entail, least of all Colon himself. They serve the dual purpose of preventing his brain from becoming overburdened with responsibility and avoiding the catastrophic possibility that he might be given a task of any real importance.
Closer examination, though, shows that Colon has some hidden depths. As Vimes thought it, most of the other watch officers saw a fat, stupid, lazy, cowardly man and that was mostly what was there, but Colon and Nobby have a street-level knowledge of Ankh-Morpork on a par with Vimes and are good at sensing tension in a crowd. Both are also survivors of the Glorious 25th of May. Colon also performs his duties in Thud! fairly well. He is an amiable jailer, and bright enough to keep the keys in a closed tin box in the bottom drawer of his desk, well out of reach of anything an inmate would be able to use. He is often portrayed as being prejudiced against women in the Watch, dwarves, undead, "foreigners" and virtually any other group that could be considered a minority in Anhk-Morpork. However his prejudice is so non-specific and naive (and frequently short-lived when actually exposed to the group in question) that nobody takes it seriously.
His office, in a separate building from the main watch house, is frequented by old acquaintances who want somewhere quiet to get away from the wife, hear what's happening on the street and - in Vimes' words - "gossip like washerwomen." For this free-flowing source of information, Vimes considers the cost of donuts on an expense voucher a very favorable trade.
He is possibly related to Sergeant Doppelpunkt (German for 'colon', as in the punctuation mark), one half of the town watch in Bad Blintz, Überwald, seen in The Amazing Maurice and his Educated Rodents. Other Discworld characters with a notable similarity to Colon include a member of the guard in the Überwald town of Bonk (who was nicknamed "Colonesque" by Samuel Vimes) and one of the market guards in Al Khali, Klatch. Like the various Dibblers, this may be due to morphic resonance.
Colon is married, though his wife works during the day; since he works at night, the two seldom see each other and instead communicate by leaving notes. Vimes even goes as far as to privately attribute the longevity of Fred's marriage to this fact. They have a son (now grown up), who is attributed to a particularly persuasive note. In Jingo, his name is misspelled as Cohen. Pratchett was probably thinking of Cohen the Barbarian.