Grey is a Fanon-color, represented by a moonstone symbol.
Grey's strategy is to fill your trash and build a powerful defense. Grey is defensive by nature, and also has defensive spells. Grey has Spirits like The AshHero Sieg-Wyvern, which become stronger the more cards you have in trash. While similar, Grey can't match White's defensive capabilities.
Themes and Flavour
The main themes of Grey are dust, ash and boundary, and has a sub-theme of chivalry. It has cards associated with dust and ash (e.g. AshHero Sieg-Wvyren), and also boundary-based Spirits, Magics and Nexus.