Colo. Rev. Stat. § 8-2-101
It is not unlawful for any two or more persons to unite, combine, or agree in any manner, to advise or encourage, by peaceable means, any persons to enter into any combination in relation to entering into or remaining in the employment of any person or corporation, or in relation to the amount of wages or compensation to be paid for labor, or for the purpose of regulating the hours of labor, or for the procuring of fair and just treatment from employers, or for the purpose of aiding and protecting their welfare and interests in any other manner not in violation of the constitution of this state or the laws made in pursuance thereof. This section shall not be so construed as to permit two or more persons, by threats of either bodily or financial injury, or by any display of force, to prevent or intimidate any other person from continuing in such employment as he may see fit, or to boycott or intimidate any employer of labor.
C.R.S. § 8-2-101
For unfair labor practices, see § 8-3-108.