Why is 초록 not acceptable when its in the list of translations?
초록 사과 is now accepted.
Why in some of these lessons when the color is used to describe a noun is 인 not used?
It's just like chocolate milk. 초록색인 사과 or milk made with chocolate is not incorrect, but we prefer having the two nouns by themselves for the sake of brevity.
Usually these are used with 색/색인 to form modifiers in a sentence.
Why 초록 doesn't count?
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.