Dual Colombian and US Citizenship - can I enter Brazil visa-free as a Colombian?

I don't know about Brazil specifically but generally speaking there is no reason this should be a problem even if these things are rarely defined explicitly anywhere. In practice, if you are asked about other nationalities (which would be unusual except perhaps if the border guard notices that you are traveling from/to the US without a visa), lying sounds like a bad idea, just tell the truth and nothing much should happen.

Two exceptions come to mind:

• You are a citizen of the country you are trying to enter. In many countries, it's mandatory to disclose that and use the local passport to enter. In countries that forbid dual-citizenship, you might even get into serious trouble if the authorities find out you have another nationality.
• Politically sensitive situations like Israel/UAE. Note that even in this case, the UAE did not recognize Israeli passports but dual citizens could still enter, only after the Assassination of Mahmoud Al-Mabhouh did they go to the trouble of actually banning Israeli citizens who also had another nationality. Typically, laws about entry are written with (relatively) simple cases in mind and if you qualify under some rule, other more restrictive rules don't matter.