There is no difference. They both mean the color green, so you can use either one. This has nothing to do with your question, but as you keep learning, you'll notice that a lot of words that have to do with the color green will contain the character 녹, such as 녹차 (green tea) or 녹두 (green mung beans) :)

Both 초록색 and 녹색 words came from the chinese character, which types you will come by more often as you study korean more. 초록색 in chinese character is 草綠色, literally means color green like a grass. 녹색(록색)綠色 literally means color green. Why did 록 becomes 녹? Those are the rules you will learn some time later. As the other people nicely put it, these two words has no difference whatsoever in usage. +)There are also korean native word 푸른색, but it is used for both green and blue color, so it’s confusing even for the natives.