Colorado voters passed the nation's first sunshine laws in 1972, with a ballot measure spearheaded by Colorado Common Cause. This act required disclosure of private interests by public officials; regulated lobbying; and -- for the first time in Colorado -- required that all meetings of two or more members of any board, committee, commission, authorities or other advisory, policy making, rulemaking, or other formally constituted bodies of any state agency or of the legislature be open to the public in accordance with the provisions laid out in this bill.
The law was revised in 1977 with the passage of House Bill No. 1018, concerning public meetings and meetings held in executive sessions. It was revised again in 1985 with the passage of House Bill No. 1097, concerning the State Board of Parole meetings. And in the 1996 session of the Colorado Legislature, House Bill 1314 - "a bill for an act concerning the open meetings provisions of the Colorado Sunshine Act of 1972", was passed.
To see the current open meetings laws, find C.R.S. 24-6-101 through C.R.S. 24-6-402. Access to Colorado Statutes and text of the Colorado Constitution can be found here.
The Colorado Press Association puts out a guide about the Colorado Sunshine Laws. Here is the 2013 edition.
Colorado Ethics Watch works to improve transparency in government as well. Read their recent report about the Colorado Open Records Act, and suggested improvements.