Otherworldy deserts of ash coming in many colours, isolated fields of volcanic grass, lakes of sulfury water as blue as a lagoon on a paradise world. Silvery seas of liquid metals (we already got that at least), volcanic ash creating fake snow, and lets not forget the polar regions which we have. I wonder if the sky looks as weird there too. Imagine snow reflecting reds and oranges.
And the sky would be really beautifull too, looking on fire, clouds of exotic elements and volcanic residue carried by strong winds, mixing together ash grey and orange.
Sunsets of weak sun that is Caldaster might sometimes turn said clouds more pinkish, and more spectacular moonsets of Prometheus, passing ever closer to the planet each year. And on the rare occasion the clouds clear people of Nocturne can see the red nebulae that surrounds the system.
Since the system is strong in radiation, there should be many strong polar lights aswell.
Basically am i wrong in imagining Nocturne as much more barren iceland? I think a un-unique volcanic world would be a bit boring, especially for Vulkan, one of the few primarchs who can really appreciate natural beauty.