In Japanese, "青" (ao) traditionally covered certain shades of both blue and green (blue sky, blue traffic lights, blue apples and many other plants).

"緑" (midori) is more solidly green, coming from a root word used to describe flourishing trees. I think that it's still considered a shade of ao. They'll sometimes use a phonetic rendering of English "green", and I'm not really clear on the difference in meaning there.