First of all what is the status of your Fiance? Is she currently in the USA or does she have a visitor visa and plans to come later?
If she is already here, and if when she originally came, she was not planning to marry you. Then you may be able to take a risky move, and marry her, and apply for her to stay. I say risky because, immigration will review the case carefully, and if they suspect that she intentionally “gamed” immigration by coming on a visitor visit, in an attempt to use it to marry you and remain permanently in the states, then the consequences may be that they will deny her request to stay, and deport her. Many couples have married, and could remain. But the best scenarios are those that did not know each other before the alien came to America to visit for touring, school, business or what not, then they met, fell in love, married. The essential aspect is that there was no plan, no intention to marry.
Now if she is not already here, it is already clear that you plan to marry. So definitely she should not attempt to come to the USA on a visitor visa and marry you. Not if she hopes to remain. She can marry you and return to Colombia, that is ok. But not marry you and try to stay.
The proper approach to bring in a fiance or spouse is to apply to immigration for a Fiance or Spousal Marriage visa. The process for either is taking 6 to 8 months. If you hope to marry here, you apply for the fiance visa. If you marry first in Colombia you apply for the marriage visa.
Once she arrives on the proper visa, she will be apply to USCISy for permanent residency.
The Fiance visa terms, are she marries you within 90 days of arrival. Then she applies for her Green Card. Three years later she can apply for US citizenship.
There are two different marriage visas. The more attractive one today is CR-1. When you apply for it, it includes the granting of the Green Card. So when she arrives in the states, she is already in possession of the Green Card. Three years later she can apply for US citizenship.
