Robins new-boy Marc Goodfellow continued his great start for the club by grabbing the only goal of the game - his third in four matches for Danny Wilson's men.
The former Stoke man powered the ball home on the stroke of half time, after good work from Lee Miller.
Colchester's evening was compounded late on when Craig Fagan was dismissed for a second yellow card.
Port Vale suffered a heavy home loss to Hartlepool United and the 5-2 success takes the Teessiders above the Potteries outfit in the table.
On-loan star Chris Shuker put his side ahead after just six minutes, but Stephen Brooker levelled matters within a couple of minutes.
Sam Collins was then dismissed for a professional foul (23) and Hartlepool took full advantage of their extra man.
Richie Humphreys made sure Hartlepool were ahead at the interval when he struck on the half hour, before second half strikes from Michael Nelson (68), Eifion Williams (74) and Darrel Clarke (83) secured an emphatic victory.
Vale did grab a late consolation through Michael Cummins (87), but it was too little too late for Brian Horton's men.
Port Vale's loss also allowed Barnsley to move above them in the table, after they earned a 3-0 success over Blackpool at Oakwell.
Daniel Nardiello (pictured) - making his debut after signing on loan from Manchester United - wasted little time in finding his feet as he pounced after just 11 minutes.
Another recent acquisition Mark Stallard doubled Barnsley lead after 54 minutes, before Nardiello grabbed another on the hour mark.
Tranmere superb run continued after a hard fought 3-2 win over Stockport County at Prenton Park.
Highly-rated Ryan Taylor put the home side ahead from the penalty spot after just five minutes, following a foul by Jim Goodwin on Eugene Dadi.
Northern Ireland international Danny Griffin the levelled for The Hatters with a well placed drive from the edge of the area.
Rovers went ahead again when Goodwin's night got even worse as he put through his own net just after the hour.
Fraser McLachlan then struck for Stockport with just 17 minutes left and looked to have earned Sammy McIlroy's relegation threatened side a share of the spoils.
However, with just three minutes remaining Anthony Williams brought down Gary Jones in the area and Taylor was on hand to slot home his second penalty of the game.
Click below for match summaries:
Barnsley 3-0 Blackpool
Bristol City 1-0 Colchester
Port Vale 2-5 Hartlepool
Tranmere 3-2 Stockport
