Colors are adjectives. As an "i" adjective, when used prenominally it must be "kuroi" rather than "kuro." However not all follow that pattern. See below: White: shiro(i) 白　（白い） Red: aka(i)　赤　（赤い） Blue: ao(i)　青　（青い） Yellow: kiiro(i)　黄色　（黄色い） Green: midori　緑 Brown: chairo(i)　茶色　（茶色い） Gray: haiiro　灰色 Light Blue: mizuiro　水色 Bright: hade(na)　派手　（派手な） Dark: kurai　暗い

kuro = a noun kuroi = an adjective