Travel to Colombia as a dual Colombian-US citizen: is it ok to use my US passport?

As stated in Timatic, the database used by airlines:

Nationals of Colombia are allowed to enter with an expired passport.

So you can enter Colombia with your expired Colombian passport, but, according to Colombian immigration (whom I just called, as I know Spanish) you'll need a new one, or an ID card (cédula) to exit the country

If you are correct that you are a dual citizen, then you need a Columbian passport (and also, if the embassy is to be believed, an ID card). From the frequently asked questions page of the Colombian Embassy to the US:

If I am traveling to Colombia and I have dual citizenship, must I enter Colombia using my Colombian passport?

Yes. Colombian law requires that all Colombian nationals identify themselves as such by presenting a Colombian national ID card and a Colombian passport when entering the country.

However, if you were born before July 4, 1991, you may not be a dual citizen. The constitution in effect at that time prohibited dual citizenship for Colombians, according to the same page:

I was born in Colombia, but I have acquired U.S. citizenship. Can I enter Colombia using my U.S. passport?

If you obtained American citizenship before July 4, 1991, under Colombian law you are no longer considered a Colombian national due to the fact that, under the Colombian Constitution of 1886 — which was valid until 1991— holding dual nationality was prohibited. Therefore, you must enter Colombia with your U.S. passport. (In this case, we encourage you carry a copy of your naturalization letter.) Be advised that if you enter Colombia with a U.S. passport you will be subject to Colombian immigration laws.

Please note that those who obtained American citizenship after July 4, 1991 did not lose their Colombian nationality and therefore hold dual nationality.

Even if you were born before July 4, 1991, however, you might be a dual citizen. I was unable to find an English translation of the 1886 constitution, or even an electronic copy of the Spanish text for machine translation, but I note the words "if you obtained American citizenship." Some countries provide for the automatic loss of nationality of anyone obtaining another nationality, but such a provision often does not apply to those who are born with multiple nationalities. Such countries may also have other provisions that lead to loss of nationality at a later time (the age of majority, for example, or some time after the age of majority is reached) or under certain conditions (depending on whether the person resides abroad, for example).

I do not know Columbian law on this matter, but if you were born before July 4, 1991, you may want to investigate further to determine whether you are still a Colombian citizen. The easiest way to determine this might be to ask at the consulate or embassy in connection with your potential application for a new passport.