Andy Burns wrote:

that's the conclusion I've come to; use a 3-pin
LED, connect the green anode to pin2 and red anode to pin4, and the
common cathode to a ground I can borrow from elsewhere (e.g. the HD LED
cathode)

then the green side will stay on all the time, the red will turn on when
the system is sleeping or shutdown, thereby mixing to give yellow, so
I'll get green or yellow

I found a 3-pin amber + green LED in the parts box, and yes it works as
above, except amber+green is not very distinguishable from green alone,
so I'll buy a 3-pin red + green LED