A lot of the people I talk to use the words "green" and "sustainable" interchangeably. If I'm tired, occasionally I do it myself. But the difference between the two is significant. One's subjective and relatively easy (although hardly the path of least resistance); the other's objective and downright difficult.
"Green" is subjective. Being "green" is modifying your societally-defined default behaviors in the hope of being somewhat less destructive of the environment and the planet. "Green" means using slightly less energy, consuming slightly fewer resources, emitting slightly less greenhouse gas, polluting slightly less water or air or earth. Being "green" can be seen as fashionable or otherwise high-status, or it can be seen as taking personal responsibility for one's impact on the planet.
"Sustainable" means just what it says. Something's sustainable if it can continue. Forever. Being sustainable means not consuming resources that exist in finite quantities, and only using non-finite resources in quantities that can be regenerated in the time it takes to use them. Using fossil fuels (which, in human time-scales, don't regenerate at all) at any level is not sustainable. Using biofuels faster than they regrow is not sustainable. (Indeed, perhaps the most common factor in the disappearance of prior civilizations is their overuse of what we now call biofuels.) Farming in ways that poison the earth or the water (neither of which regenerates, in practical terms) is not sustainable. At the same time, supporting a family by hunting and gathering is becoming unsustainable, due to environmental degradation (the good hunting and gathering lands are getting fewer and farther between). Living in a way that truly sustains the environment, in today's North America, is practically impossible.
When I talk to students and faculty, I make this distinction as clearly as I can. If higher education has a role to play in making society actually sustainable, it involves behavioral changes and technology improvements that aren't on today's radar screens at all. Our educational role either succeeds or fails in the long term.
When I talk to administrators and staff on campus, I place less emphasis on true sustainability. Staff get evaluated on what they accomplish in the short term, and in the short term sustainability just isn't going to happen. Staff can increase our recycling rate, decrease our energy utilization, cut down on our resource consumption. But staff can't make any of those problems go away. There's no point in making people feel guilty when they're doing the best they can. (If they're not, then maybe guilt has at least the potential to be a productive emotion. Or not.)
So "green" means taking the currently available baby step on the journey towards a society which eventually can live within its resource constraints. And a key job of education is to find ways to free up those constraints not (as we've done in the past) by finding other finite resources to exploit, but by finding ways not to depend on the exploitation of finite resources at all. And to exploit renewable resources only at rates slower than those resources regenerate.
It's definitely a two-track conversation. (And occasionally, I find myself on the wrong track.)