Can an employer in Colorado refuse to pay an employee for vacation time earned by an employee?

Yes. There is no law that says that an employer actually has to give you any paid vacation in the first place. A vacation, like sick leave, is a perquisite of the job.

That's the reason that you must work for a minimum number of days before you are eligible for any time-off.

If you resign or are terminated before you take your "earned" vacation, unless the employer chooses to compensate you for it, you simply forfeit the time off and any equivelant in pay because you did not take advantage of the perk attached to the job.

As always, there can be exceptions (e.g.: if your employment is covered under a specifically Worded contract).

For further information contact your State Dept of Labor.