Common-law marriage is one of the most misunderstood legal topics. Available in only a handful of states, common-law marriage is not a different type of marriage. Rather, it is simply a different way of forming a marriage. Once two people marry - whether by common-law agreement or ceremonial marriage - they are married for all purposes until the marriage is ended through divorce.
Common-law marriage is similar to a verbal contract. A verbal contract is every bit as binding (for most things) as a written contract - it is just harder to prove that it exists when there is a dispute. Similarly, a common-law marriage is every bit as valid and binding as a ceremonial marriage entered into with a marriage license - it is just harder to prove that it exists when there is a dispute.
A common-law marriage in Colorado requires 2 things. First, you must cohabitate (live together) as husband and wife. Second, you must agree to be married. If you and the person you are living with both agree that you are married - then you are married. There is also some authority that indicates that you must hold yourself out to others as husband and wife, but the authority is somewhat unclear whether this is an actual requirement or simply a way of establishing that an agreement existed.
Problems often arise with proving that the common-law marriage exists. When this issue comes to court, it is often in one person's interest to deny that they ever agreed to be married and in the other person's interest for there to have been a marriage. Such disputes can be overcome through proof of the agreement. One type of proof is "holding out" or the reputation of the parties in the community. If you each refer to each other as husband and wife and all your family and friends believe that you are married, then this is good evidence that you are actually married. Signing documents as "husband" or "wife" is also pretty good evidence. Other evidence can be even stronger. Listing someone as a spouse on medical insurance is strong evidence because in most instances making a false statement on such documents constitutes insurance fraud. Filing joint tax returns is also very powerful, because if you list someone as your spouse and you are lying, you have committed tax fraud. If a party signs such a document and later wants to deny the existence of a marriage, they will essentially be forced to admit to falsifying insurance or tax documents. Owning a vehicle or a house together, on the other hand, is not particularly strong evidence because people often own property jointly with people they are not married to. Filing separate tax returns provides some evidence that you did not believe that you were married. Further, evidence that you are planning a wedding in the future supports a conclusion that you have not formed a common-law marriage at the present time. Remember that if you are married, you are married for ALL purposes. Except for how the marriage is formed, there is no legal difference between common-law marriage and ceremonial marriage. Further, there is no "common-law divorce". Therefore, if you are married, you need to consult with a divorce attorney.