A majority of Colorado citizens had voted in favor of Amendment Two which prevented any city, town or county in the state from taking any legislative, executive, or judicial action to recognize gay and lesbian citizens as a protected class. An injunction against the amendment was issued by a state court. The matter went back and forth between the Colorado Supreme Court and the lower state court before being heard by the US Supreme Court. On May 20th, 1996, the Supreme Court ruled 6 - 3 to strike down the state amendment. The SCOTUS's decision would set the stage later when it would hear Lawrence vs Texas.
Learn more about Amendment Two and Romer vs Evans by reading the Lambda Legal article, Wikipedia's entry and Oyez.org's record.