[patton]

Main engines can't be pure H2 + O2 - you can see the fire.

[Ichneumon]

Main engines can't be pure H2 + O2 - you can see the fire.

I'm not sure where you get the idea that an H2/O2 flame won't be visibile under any conditions, but that is indeed what the Shuttle's main engines use for fuel.

Here's a hydrogen flame -- looks pretty visible to me:

[Ben Hecks]

Main engines can't be pure H2 + O2 - you can see the fire.

This is just a guess but the liquid rocket engine nozzles are coated with an ablative material, some of which burns off during flight. It's possible that the residue from this is visible.