青, あお, ao, blue (noun). 青い, あおい, aoi, blue (adjective). COLORS (ADJECTIVE) AKAI - Red AOI - Blue SHIROI - White KUROI - Black CHAIROI - Brown KIIROI – Yellow COLORS (NOUN) AKA - Red AO - Blue SHIRO - White KURO - Black CHAIRO - Brown KIIRO – Yellow 1) 黄色 はきれい。- kiiro wa kirei desu. - Yellow is pretty. (this is a noun) 2) 黄色い とりはきれいです。 - kiiroi tori wa kirei desu. - The yellow bird is pretty. (this is an adjective) 3) 黒いふくが好きじゃない。- kuroi fuku ga suki ja nai. - I don't like black clothes. (this is an adjective)

this should help http://www.italki.com/answers/question/91441.htm

ao is noun blue color aoi is adjective blue also siro and siroi most japanese adjective ends in 'i'