Another area that has been left to the states is the protection of a worker's right not to join a union. Some states have a “right-to-work” law that prohibits compulsory union membership; Colorado is not among them. Colorado prohibits union contracts calling for closed shops (in which only union members may be hired), but it allows union shops (in which new hires must join the union within some specified period of time) and similar union security agreements, as long as these arrangements are properly ratified by employees. The law also provides for the periodic reconsideration of union security agreements by employees. Like most states, Colorado does not allow employers to deduct union dues from a worker's wages without the worker's individual authorization (CO Code Sec. 8-9-106).
"Yellow Dog" Contracts. An agreement in which an employer requires an employee not to join a union during employment or face discharge - commonly known as a "yellow dog" contract - is against public policy (CO Code Sec. 8-3-119).