Thank you for the photo of the wiring on your trailer.
Wire colors alone are not a reliable way to figure out what signal the wire carries but heavier gauge wires are often used for the trailer's main ground (usually a white wire), for 12V auxiliary power for the trailer or for the brake circuit on the trailer (if it has electric brakes) for which usually a blue wire is used.
You can determine what each wire does either by tracing them back to the lights or other things they connect to... view full answer...