If you have an employment contract (written) which specifies which state's law to apply, apply that state's law: such "choice of law" provisions in contracts are legal and enforceable.If not, then if you work in CO, CO law would apply; each state applies its own employment laws to people working within it, regardless of where the company is headquartered or based (so long as, again, there is no contract to the contrary). As you note, CO requires the payment of accrued but unused PTO on separation from employment; here is a link to a CO government website on the subject: https://www.colorado.gov/pacific/cdle/vacation