According to studies that I have seen, we are conditioned to react with disgust against certain colours that occur in nature (such as green and yellow) in certain circumstances (particularly, as
mark 12_30 says, when they are slimy or sticky green), because they are associated with disease. For example, we are less likely to want to touch a bowl of sticky green liquid than a bowl of sticky blue liquid. Blue, since it occurs rarely in nature, does not evoke such a strong reaction.
Here is a fun test which makes the point.
Of course, green also represents vitality, again because it is a natural colour. So, we are told to “eat our greens” because they are good for us. And I am sure that I am not alone in deriving great pleasure from the image of sunlight on leafy trees. Also, in international signage, green represents "go" or safety, whereas red represents "stop" or danger.
So it is ambivalent, but it’s more likely to provoke a bad reaction in situations where we don’t expect it. Also, the shade will make a difference, so the pale dull green of Minas Morgul (as captured well, I think in the RotK film) will seem threatening. But I imagine the green of Rohan’s banners and Bilbo’s door as a pleasant, vibrant green.
Green can also represent danger, in that it is used by some predators (amongst the reptilian and insect worlds) as camouflage. Perhaps Old Man Willow falls into this category.
