Wiki: The Gallagher Amendment was an amendment to the Colorado Constitution enacted in 1982 concerning property tax. It set forth the guidelines in the Colorado Constitution for determining the actual value of property and the valuation for assessment of such property.
Here’s a interesting recent Denver Post story that talks about this amendment: http://www.denverpost.com/2017/01/17/hickenlooper-pot-sales-tax-hike-school-funding/