If you are using an LED with less than 20mA supply current, you can use any (preferable a XOR GATE in this situation) Logic Gate. Simply wire the cathode to the output of the LED and wire the Anode of the green LED to the Output of the same logic gate. When the switch is on, it will show as green, supplying the Green LED with current, when the Switch is tripped, it would allow the Logic gate to 'sink' the current of the Red LED.
The input to the Red LED would be just powered from 5 volts or whatever voltage you have, with respect to the proper resistor in line to supply the proper forward voltage of the LED.
The Hall Effect Sensor output would got to one of the A or B inputs of the Logic Gate, (instead of the Anode of the LED as before), and the other input of the logic gate would be connected to ground. Most CMOS Logic Gates will Sink or Source up to 25mA, any more than that, you'll destroy the logic gate. But Your LED's will most Likely reach enough (if not full) brightness at around 10mA anyway.