The Eternal realms are beyond mortal ken.
The Eternal realms are home to most of the gods, whom are said to have their thrones in and around the Axis Mundi, variously portrayed as a great tree or a great mountain. The deities of Kur are said to be enthroned in the roots, the deities of Elyseum round the trunk, and so forth.
The Eternal realms are home to the highest daimons that ever interact with mortals. It is said that Kur is the home of the severe (many would say downright malevolent) daimon princes, whereas Elyseum is the home of the merciful daimon princes.
Known also as Hades, The Land of the Dead, Hell or Helheim.
It is said that to enter Kur from Harmony, one must pass through seven gates. Dead souls may pass freely, but those who would enter must know the gates' names and rituals. Those so knowledgable will also know that for each gate, one must leave part of oneself.
Kurnugi gives the impression of a vast realm of darkness, dust and shadow. Vast rivers of ice and fire run through it, and the depths of its crevasses and heights of its mountains dwarf anything in Materia.
All dead souls are judged in the courts of Kurnugi, and pass from the courts to torment or reward. Some souls are tormented for a lifetime or more by devilish daimons whom make their home here.
Some are allowed to pass through the twelve doors at the far end of Hell, and from there unto Elyseum.
Hades interfaces with the Harmony, Elyseum and Tarterus.
Tarterus is the husk of Kur. It is the depths underneath hell wherein old gods are imprisoned and rebellious daimons hide. It is said that through Tarterus, Kur is assailed continually by Chaos. Tarterus is rage and senseless destruction.
Only the greatest heroes have ever reached Elyseum without dying. The Elysian fields are the boon of the virtuous dead, those who have been refined sufficiently in the worlds of Matter and Spirit to pass the judgements of Kur and transcend mortality forever.
Little is known of Elyseum.
These are the final three realms, known only to the gods and their intimate servants. It is said that no mortal has ever reached these realms. However, among the most ambitious of sorcerers, it is claimed that the truly great may achieve these realms, but if this is done, one becomes like unto a god, and has no tethers to bring one back to the Materia ever again.
The Realm of Wisdom would presumably be associated with air and the colour grey, the Realm of Will with earth and the colour black, and the Realm of Infinity with Aether and the colour white.