You are right that they both mean the green color. The difference is very small. 녹색 is a more general term for the color, as when you compare different colors. 초록색 on the other hand adds the image associated with the green in nature. 초 is a hanja character which means plants or verdure, and 록 is the same character as 녹 (it becomes 녹 if it comes at the beginning of a word), so it emphasizes the fresh and natural green. For example, there is a children's song called 초록 바다 (" 초록빛 바닷물에 두 손을 담그면 ..."). In a song or poem, 초록 usually works better than the dry 녹색 as it evokes the sense of nature's regeneration ( Note that 초록색 is also referred to as 초록빛 or just 초록).

both same. 녹색=綠色,came from chinese language. 녹(green)색(color)