Three Things form this World and Cosmos: the Matter, the Spirit and the Essence. The Sun and the Void rule the Essence, the Blue Moon rules the Spirit and the Red Moon rules the Matter. Everything under the Iron Moon has a chaos of mass, a pattern of form and place and varies during time.
From the surface of Urd, Rijk looks the greatest of the two moons. It is, in fact, bigger than Ura, and it orbits the planet farther than its sister. It normally looks red or maroon in the sky, like rusted iron, thus the people in Urd call it "the Red Moon", "the Iron Moon" or even "the Blood Moon".
Rijk tidal influence affects anything in Urd: water, living bodies, rocks and even the atmosphere. Its presence in the nightsky make people feel different, everything changes its behavior: from the mice and grass to the sea and wind. It is a moon that brings happiness, but also anger and destruction.
Rijk goes from full moon to new moon in 27 days, and spends around 5 days in each phase, that's why it is used to calculate months and weeks in Urd.
When it is in full phase, and alone in the sky, people of Urd talk about a "Red Night", name of one of the Seven Schools of Magic. Those nights earthquakes, storms and other natural disasters are typical, that's why it designates the school dedicated to alchemy and elements.
When it is in phase different from full or new, and Ura is near in its full phase, the people talk about a "Blue Night", which is one of the Seven Schools of Magic too. Since in those nights strange visions and omens are normal, it designates the school dedicated to divination.
When both Ura and Rijk are in full phase, it is called a "Purple Night", and those nights the magic become loose, making supernatural and natural disasters normal. That's why it designates the School of Magic dedicated to hunting monsters, fairies, rogue wizards and the undead.
And when both moons are in their dark phase, and the sky looks empty, people in Urd talk about a "Black Night", the name that applies to the only school that still uses necromancy in order to talk to spirits.