Under Colorado case law and page three of the Colorado Landlord Tenant Handbook, unless the lease prohibits subleasing, a landlord may not unreasonably withhold permission to sublet.
There are no Colorado statutes or guidance for what kind of refusal is reasonable, so you can follow the general rule of thumb: an applicant can only be reasonably refused if it can be proven that they would put the landlord's business at risk. If there is no evidence that they would have the ability to pay the rent reliably as a subtenant then there are grounds for reasonable refusal.
The Federal Fair Housing Act and the Colorado Fair Housing Act prohibit discrimination on the basis of race, color, creed, religion, national origin, ancestry, sex, disability, familial status or marital status. A landlord cannot refuse to rent or negotiate different terms based on these characteristics.
Colorado does make an exception here for people renting rooms in single-family, owner occupied homes. It also makes an exception for religious organizations, which are allowed to give preference to individuals of the same religion, and private clubs wanting to give preference to its own members.
The information provided on this website does not, and is not intended to, constitute legal advice.