While looking for an answer to this question myself, I came across [this article][1] which talks about actual studies that have been done regarding the colour of plastic greenhouse covers. In particular it states:

> In studies, it has been found that using green plastic to cover a greenhouse results in plants which are slightly (but very slightly) shorter than plants grown in a greenhouse with a clear plastic covering.
There isn’t all that much green in sunlight, it seems, since overall, green plastic lets in almost the same amount of light overall. It’s probably not preferable to clear plastic as a greenhouse covering, but if you find yourself in the unlikely situation of having only green plastic sheeting available for your greenhouse, it will do if need be.


  [1]: http://www.greenhousedeals.net/how-to/what-are-the-best-colors-of-plastic-to-use-for-an-outdoor-greenhouse/2013/04/11/