The Red External LED mirrors the true Arming LED (that you can see through the Flight-Controller's white plastic case). Obviously, if the true one isn't lighting, neither will this external one.
The Green External LED mirrors the (tiny red) true GPS-Fix LED (on the bottom of the GPS-Module).
There is also a yellow LED that signifies various calibration modes.
The pinouts for the 4-pin black DuPont External LED assembly in here.
To test LEDs (to make sure a non-lighting one isn't burnt-out): After documenting plug orientation, you can disconnect it from FC. Then apply (properly polarized) 5 volts on the proper 2-pins sets ... to test each external LED. Do not reverse polarity or you might blow an LED.
JW57RC (in RCGroups forum) manually traced-out his particular LED assembly, and created this schematic