I have wired 2 flood lights outside and believe everything is in proper order, wires from both lights go to a junction box and are then wired with the main line and a line running to the switch, all colors matched, then the single pole switch is wired properly.  when turning on the breaker, the lights are on, but when flipping the switch to the off position, the breaker trips.  where could my error be?

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but when flipping the switch to the off position,

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Suspect that that is incorrect. Suspect that you are turning the switch to the ON position.

Line goes from breaker to junction box, from there it goes to each light with a switch loop to switch.

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all colors matched

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At the junction box the wiring should be,

Black wire of the incoming power cable is connected to the white of the cable going to the switch. That white wire should also be colored black (each end) to designate that it is a hot, not neutral conductor. The black of the switch cable should be connected to the two blacks going to each light fixture. The three remaining white wires should be connected together.