Ah, I see, I misread your comment quite badly. I don't have any references to look this up in, but perhaps the ancestor of the word midori is first attested in a syllabic spelling in Old Japanese with only the meaning "sprout", and when it came to be spelled logographically as 綠, it was mainly used with the meaning "green". But the question could be asked in the Tea Room of Wiktionary, where it would probably find its way to someone more knowledgeable than me.