Colorado: Taxation. Income-tax Colorado levies on income below average for the nation. ,1939 p-5 Denver survey traces income tax to its origin in 1646, Post. Sept.17,1939 19.15 Public employees must pay first income tax in 1940. Post. Dec. 37,1939 State income tax return demanded of. Guffinbagen. Post. Apr.17,1939 19.3 . State prepares to tax income of U.S. employees. ` ,1940 19.4 0