Colors

Various colors are associated with certain beings. See also the Noble Colors.

Blue
Color of those under Shinto. Judging by Nanaya Kiri's ability, there are those in the Demon Hunter Organization who follow its beliefs. Jougan are also this color. While Tohno Shiki's eyes turn blue when he uses Death Perception due to his eyes originally being Jougan, Ryougi Shiki's are also depicted with blue eyes. It is likely that not only Jougan, but the Mystic Eyes of Death Perception are things that came from the Gods.

Gold
Color representative of the demonic. Arcueid's eyes turns into this color. Also, Nero's eyes were depicted as golden.

Red
Color representative of the demonic. Judging by Nanaya Kiri's ability, those who fall under the classification of 'demon' will likely be inclined towards this color. Similar beings are also likely to possess this color: Arcueid, vampire Satsuki,  the fully-inverted Akiha, Roa (SHIKI), Len,  Ciel (Roa),  Kouma,  vampire Sion. In Zepia's case... well, it's fine either way. Even if it's caused by overflowing blood.

Silver
Color of those under Shinto. Judging by Nanaya Kiri's ability, there are those in the Demon Hunter's Association who follow Shinto beliefs. No known characters possess eyes of this color.