Bi colour led status indicator

Good day.
i have attached a circuit. Its a status indicator for a relay driven by an open collector comparator chip.
when relay is on, green led is on. And when relay is off, red led on.
“Vrel” connects to the open collector side of the relay. The other terminal of the relay itself, goes to +Vcc.
My question is what is the purpose of D10?
I have boarded the circuit and cant tell the difference with the diode in or out.

I can see any way that the diode would conduct.

It could be that it is there to protect against some external influence, like an accidental connection of one of the LED wires to a larger voltage.

However, it's entirely possible that the diode wouldn't ever achieve its intended purpose, and no-one has ever tested it. Some designer thought it would have some effect, and included it, and as it does nothing, there is no incentive to remove it.

I've seen diodes used to stop LEDs being subjected to reverse voltages, because the data sheet says that 5 V reverse is the maximum, when in practice they will survive much more, and so the diodes remain, just in case.

The schematic comes from an Elektor article here:
Pages 53 & 54.

I think that the diode was intended to prevent both LEDs being on at the same time, but that isn't needed. It could be that something else has changed.

I suggest that you ask the designer. You can contact him through https://linearaudio.nl/welcome

The two LEDs indicate when a relay is energized as explained in the article Mickser referenced. Look closely at the transistors – one is NPN, the other PNP. Vrel at the top is the voltage across the relay coil.