Impressive win for the U's youngsters.
The U's were in front within two minutes of kick-off. The ball found Dion Sembie-Ferris on the right hand side and he rifled in a shot that flew past Cornell and into the far corner.
And, just as importantly, there were ninety minutes for Ben Gordon, George Moncur, Jack Marriott, Kenny McEvoy and Rhys Healey ensuring boss Tony Humes has good options going into the weekend trip to Doncaster Rovers.
Following on from the first team's win against Bristol City on Saturday, the U21s ended Swansea's fourteen game unbeaten run to continue the U's youngsters' impressive start to 2015.
Colchester United U21s recorded an impressive win as, for the second time in three days, a U's side overturned an opposition currently topping the league table.
It was a bright start to the game for the U's, and Healey should have doubled the home side's advantage when he got in one on one with the keeper after Marriott had robbed Grimes, but Cornell stood strong to see it away.
The U's were nearly pegged back on seven minutes, when a free kick delivered into the box by Henry Jones was diverted towards goal by Kane Vincent-Young and Bransgrove saved well from his own player at close range.
After an explosive start, it quietened down a little, although Moncur had a chance on 17 minutes when a corner found him on the edge of the box. But his shot was straight at Cornell and easily saved.
From a Swansea corner five minutes later, Bransgrove pulled off a fine reaction save to turn the ball over, as the set piece was diverted towards goal from close range.
Midway through the half, though, the Swans were level. Hanley spotted the smallest of gaps at the near post to smash home a shot that flew past Bransgrove.
That said, the U's were almost back in front straight away. McEvoy found the tightest of spaces in the box to turn and shoot, but it was a fine reaction save from Cornell.
Both teams were playing some good, punchy football, and some great build-up play from the U’s just before the half hour mark saw Gordon flash a cross across the face of goal that went begging.
McEvoy and Gordon did combine after 33 minutes. The left back lifted the ball down the flank for the Spurs man to chase and he cut inside Roberts before lifting the ball over Cornell to make it 2-1.
But City were level six minutes later. A swift paced attack saw the U’s chasing back, but the ball was cut back to Gorre, who lifted it home high past Bransgrove.
The U’s almost found a third as a snap volley from Marriott was well saved by Cornell.
It was all square at the break, with City introducing Gogic for Joseph Jones at the start of the second half.
Both teams had good chances at the start of the second half, with Sembie-Ferris hitting a dipping shot just over the bar, before Bransgrove smothered a near post effort from Gorre.
The U's regained their lead nine minutes into the second half, as Healey converted from close range after good work by Marriott and Sembie-Ferris.
The home side made a double change not long after, as Sembie-Ferris and Harney came off to be replaced by Corey Galvin and Tosin Olufemi.
And the substitutes were still settling into the game when the U's extended their advantage. McEvoy got in down the left and cut it back to Healey, who moved the ball before blasting past Cornell.
They almost got another shortly afterwards as Galvin got into the box from a short corner and forced a good low save out of Cornell.
But they did get their fifth midway through the second half. A free kick into the box found Marriott in acres of space and he smashed home.
After that explosive treble of goals, the game settled down somewhat, although Swans sub Fallon stung the fingertips of Bransgrove with a shot that the U's keeper fielded well.
And it summed up an impressive display from the U's as they continue an impressive 2015 for the youngsters.
The Swans should have got a third late on when Moncur was robbed of the ball, but James did well to see off a 2-on-1 and allow Bransgrove time to come out and smother.