No ATM in Colombia will allow you to withdraw cash dollars. You can always withdraw pesos as all ATM's will give you the official exchange rate, or a rate very close to it and will not charge any fees in addition to the ones your home bank may charge you.
You can then change those pesos for cash dollars, and these cash dollars should normally be cheaper than the official exchange rate, so after exchanging you would actually have more cash dollars in hand than what your bank account showed you withdrew, unless they charge incredible fees for the withdrawal.
The problem might be that banks in Colombia will only let you withdraw max. 500.000 COP per withdrawal, and max 1.900.000 COP per day. So if you need more than that you would need to go several days to ATMs. Davivienda, which has ATMs all over the country, is the one allowing the largest withdrawal, and is the info Im giving you. Other banks let you withdraw even less.
I don't have much clue of what the current situation is in Caracas, I would suggest posting a new thread in the Venezuela forum for up-to-date info.
Remember to be careful with your money. Go to an ATM in a shopping mall and try to change your dollars in the same mall, and hang around a bit in some shops before leaving with all the cash in hand. Also, change at a trustworthy place, someone here should be able to point out a good place in Medellin or Cartagena, there are a lot of false dollars around.