2 LED Parallel circuit help?
When S1 is made, current flows through K1's coil, pulling the armature down and connecting the Common contact to the Normally Open contact.  That will allow current to flow through the green LED and its ballast resistor, lighting the LED.

When S1 is opened, current will no longer flow through K1's coil, the armature's spring will force the Common contact away from the Normally Open contact and connect it to the Normally Closed contact, lighting the red LED.