Developers have built 3,500 “green” homes in Colorado during the past two years, more than $1 billion worth, making Colorado the top green building state. Colorado kicked off the nation’s only statewide green building program in March 1997 and has assembled a task force that is raising money for public education and hopes to launch a website to share building techniques and advice with builders, suppliers, and consumers. Green buildings include more insulation, toilets and faucets that use less water, and less toxic paints, features that add about $1,000 to $2,000 to the price of each home — money that can be easily recouped in energy savings over three to five years. Colorado is one of six pilot areas for a Fannie Mae program unveiled last month that lets buyers borrow up to 15 percent of the cost of a home to pay for energy-saving features.