The Sky in this world. Above the gray sea of clouds, there is not a blue, but a red sky.
It's not because of a pollution, but from the blood of one of the Aristoteles that appeared near the end of the Great War, Type-Pluto.
In order to not let Pluto enter the planet's atmosphere, the Six Sisters fought it and killed each other at the same time, and its blood covered the whole planet as a result.

Notes - The Sky, it's Red. [blood sky]