Colo. Const. art. XV, § 15

Contracts with employees releasing from liability - void. It shall be unlawful for any person, company or corporation to require of its servants or employees, as a condition of their employment or otherwise, any contract or agreement, whereby such person, company or corporation shall be released or discharged from liability or responsibility on account of personal injuries received by such servants or employees while in the service of such person, company or corporation, by reason of the negligence of such person, company or corporation, or the agents or employees thereof, and such contracts shall be absolutely null and void.