Goals from Kevin Gallen and Tony Thorpe were enough to see off Colchester, who had Karl Duguid sent off.
The visitors were a goal down and struggling when Duguid was red carded in the 32nd minute after stamping on Marc Bircham.
Earlier, Gallen had given Rangers a 12th minute lead by scoring from close range after centre back Daniel Shittu stormed down the left and pulled the ball back from the byline.
Things got worse for the U's when they lost Scott McGleish to a head injury although his replacement, Wayne Andrews, immediately had the chance to equalise when he raced clear of Terrell Forbes only to overrun the ball with keeper Chris Day to beat.
Duguid's exit left Colchester with a mountain to climb. But they made a decent start to the second half and Kem Izzet fired over Day's bar after being teed up by Sam Stockley.
Rangers went further ahead in the 57th minute. Defender Alan White misjudged Day's long clearance and Thorpe nipped in to head the ball over keeper Simon Brown into the net.
There was no way back for Phil Parkinson's men and Rangers might have added a third.
Brown saved Kevin McLeod's weak shot and kept out a Gallen volley after the R's striker had gathered Thorpe's header on.
Colchester finally managed a shot on target eight minutes from time, when Day did well to hold Rowan Vine's effort.
But Rangers were comfortable winners and are now five points clear of third placed Bristol City.
On a less positive note for manager Ian Holloway, midfielder Marc Bircham limped off near the end with what appeared to be a knee injury after a crunching tackle from Craig Fagan.
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