So the heating element in these heated beds do not include any type of a diode. They get warmed by current flowing across a constrained path. And that resistance will build the same amount of heat no matter which way the current flows over it. Why there are red and black wires going to the heat bed idk, but 2 reds would be just fine. Now external mosfets very much care about that polarity coming in. If you are wiring the heat bed output from a mainboard into a external mosfet then you need to ensure that polarity is right. If you are wired directly from the board to the bed and connections are correct and tight and the bed doesnt heat up then you should look in marlin under configuration.h and make sure that the heated bed is enabled. And once you get it all up and running don't forget that PID autotune for them heaters.