There's this girl and was wondering if I could legally kiss/makeout with her in Colorado

Yes there us no age limit for that I just advise not to do it in public

Laws vary by state (and are changing a lot these days) but they are pertain to sexual intercourse only. The trouble is that any angry adult can make your life miserable if they want to involve the police, school administrators, and the girl's parents, just because they think you are having sex.
Here's part of the CO statute that addresses sex between minors and what IS a crime. You can Google Romeo and Juliet laws for general information across the country.

(d) At the time of the commission of the act, the victim is less than fifteen years of age and the actor is at least four years older than the victim and is not the spouse of the victim; or

(e) At the time of the commission of the act, the victim is at least fifteen years of age but less than seventeen years of age and the actor is at least ten years older than the victim and is not the spouse of the victim; or

18-3-405. Sexual assault on a child.
Statute text

(1) Any actor who knowingly subjects another not his or her spouse to any sexual contact commits sexual assault on a child if the victim is less than fifteen years of age and the actor is at least four years older than the victim.

Depends on how you define "Making out". Thirteen is generally too young for ANY intimacy. The more important question is do her parents allow her to date?

Thanks very helpful