Theo wrote:

The Intel doc only describes green and yellow states, never red.

I think they're assuming and amber+green LED, that's certainly what's
onboard, you can see the individual dies on the LED, and it isn't mixing
red plus green to get yellow, whereas I have a red+green LED.

The manual says:

FP PWR_P/SLP_N: Power/Sleep messaging LED terminal 1 with 510Ω pull-up
resistor to +5V_A voltage. Connect it to an extremity of a dual-color power
LED for power ON/OF, sleep and message waiting signaling. Please refer to Intel®
Front Panel I/O connectivity Design Guide, chapter 2.2.4, for LED
functionalities and signal meaning.

It seems they're using the intel physical connector, but with different
meanings for the signals.

implies an R-G LED with separate ground, rather than an Intel
back-to-back G-Y LED.

Yes despite the doc, 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, rather than green or red

Poor documentation though ...