With all sorts of vehicles touting fuel efficiency, low emissions, and a dizzying array of alternative fuels, what's a person to do? If you need a vehicle, how do you find the best one that does what you need but doesn't do in the planet? The answers aren't so clear-cut (so to speak).

Want a hybrid car? There are full hybrids and mild hybrids. The Toyota Prius, a full hybrid, connects an electric motor and a battery to a standard combustion engine. A mild hybrid, like the Honda Insight, works the electric motor only when extra power is needed. This can make it slightly more efficient than a regular vehicle in stop-and-go traffic, but it can't run on battery power alone and doesn't offer the same fuel efficiency as a full hybrid.

Then there are plug-in hybrids, which are out on the roads but not yet mass produced. A plug-in hybrid has a battery pack that can power the vehicle for up to 60 miles - further than most people drive in a day. If you do have a longer trip, the engine kicks in automatically after the battery runs out of juice. You can recharge the battery using an ordinary 120-volt socket.

Let's not forget about the whole swath of alternative fuel vehicles, which can be a murkier area for a lot of people.

Efficient and cleaner diesel vehicles - which can compete with full hybrids in terms of fuel efficiency - have taken off in Europe. According to Business Week, 50 percent of all vehicles sold in Europe are diesel-powered, by only 1.5 percent of U.S. vehicles are diesel. However, a study done by JD Power & Associates predicts that number to double by 2012. Volkswagen, Mercedes-Benz, Ford, and DaimlerChrysler all expect to sell diesel vehicles in all 50 U.S. states by the end of 2008.

There are about 3.5 million flex-fuel vehicles on the road right now, all of which can run on a blend of 15 percent gasoline and 85 percent ethanol, or E85. It's cleaner burning than straight gasoline, and helps strengthen rural economies. But many people don't even know whether their vehicles can handle E85, or don't have easy access to E85 pumps. Infrastructure continues to be an issue, and there's a lot of talk that we'll soon see ethanol made from a more efficient plant source than corn.

For better or for worse, there's no easy answer when it comes to "Which car is the greenest?" Whether it's hybrid, diesel, a plug-in, or ethanol, you'll want to do your homework when determining which shade of green is best for you. Click here for some of the latest models.

about.com
Business Week
Edmunds.com
National Ethanol Vehicle Coalition
Plug-In Partners