Why is 초록 not acceptable when its in the list of translations?

Why 초록 doesn't count?

초록(+색), 보라(+색)/녹(+색)

The pattern I've noticed is that a noun is preceded by an adjective (프른, 노란, 빨간) or color+색. In the latter case, the color is in noun form, but it's a noun being used to describe another noun.