Green Paint is water-based and soluble (latex). Paint that is bad for the environment is made with solvents, oil, or petroleum, and is not soluble (alkyds). It can be a big problem to dispose of unused alkyds-based paint. Many contractors use oil-based paint, so it is best to make sure they will use only latex before you you commit to using their services.

You can find an excellent article by Bob Vila that explains the differences, pros and cons, between the two types of paints at http://www.bobvila.com/HowTo_Library/Selecting_the_Right_Paint_Type-Painting-A1953.html