So is it AO or AOI?
The noun forms are listed.
ao (noun)
aoi (adjective form)
青が好きです。
ao ga suki desu.
I like blue.
青い車が好きです。
aoi kuruma ga suki desu.
I like blue cars.
Color words in Japanese.
YELLOW黄色 ki iro
BLUE青 ao
RED赤 aka
WHITE白 shiro
BLACK黒 kuro
GREY灰色 hai iro the color of ashes
PURPLE紫 murasaki
GREEN緑 midori
BROWN 茶色 chairo