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

You can use any of our passport to enter a country. But you will need to leave the country with the same passport you entered it with.

Is there a proper reason you want to leave Brazil on your Canadian passport?

Keep in mind that even if you leave Brazil on your Colombian passport, you can still enter whatever country you are heading to on your Canadian one. So even if you go back to Canada, you will need to leave Brazil on your Colombian passport, but that is perfectly fine.

You might want to show the airline both passports, so they know you are allowed to go to Canada.

While MrTweek is technically right, from my own experience in Central America (South America was mixed) and from what I have heard from many other travelers (I spent about 11 months backpacking in Latin America last year) you might get stuck with the passport you use if you exit/enter by LAND. Often immigration officers want to see your legit OLD exit stamp when you enter the NEW country by land and then only allow you to use that passport and not your other passport.

Since I am not sure if I described it correctly here a quick example:

You enter Chile by plane and use your Canadian passport. You then take a bus to Argentina and get your exit stamp in the Canadian passport. In Argentina you want to use your Colombian passport. Argentina refuses and wants to see that you exited Chile properly with your Canadian passport. It also refuses to accept your Colombian passport to enter Argentina and only stamps the Canadian one.