Be green means being friendly to the environment. That is a positive or negative phrase, depending on where you stand politically. Too often, 'be green' is actually an empty phrase, without any real meaning. Companies say they are 'going green' when all the do is change the color of their advertizing to green. A better phrase to me is 'sustainable', because that can be measured objectively. To find out if what you're doing is environmentally sustainable, simply ask yourself "Can we be doing this same thing in 200 years?"

I would agree with the first poster, I wouldn't say it's a negative term at all. Some people may be skeptical that when someone says they're "being green" whether they're actually being useful to the environment or not, but that doesn't make it a negative phrase, I wouldn't say. Now there's another phrase "turning green" which means the act of getting seasick or otherwise sick to one's stomach. I would say that has a bad connotation, but that's a different phrase.

If you are asking about grammar then the meaning is positive. It is telling you to do something. It is not telling you not do something. Whether you think 'being green' is a positive or negative thing if another story. I would say that it is also being positive (as in encouraging good) 'to be green' ie environmentally aware.

^ everyone else has explained this well, but I thought I'd add a list: "green" could mean a variety of things: -Nature-friendly/good for the environment -jealous/envious "green with envy" -nauseous/seasick -inexperienced -or... the color green ^_^

I'd interpret it as "be environmentally aware", so it's a positive message. Perhaps you suspected "green" to mean "envious", but we wouldn't make such a connection in that context. It doesn't make sense to tell people to be envious. The intention should be very obvious from the context.