why do we use "i" after colors like "kuro" kuroi
So why? and when do we use it ?

and with all colors ? or few ?

Question

why do we use "i" after colors like "kuro" > kuroi
So why? and when do we use it ?

and with all colors ? or few ?

Answer

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

Answer

kuro = a noun  kuroi = an adjective