It's better you don't bring cash to the island to change it here. The exchange rate is really bad here, spezial Euro / Peso.
You can get cash at an international rate at every ATM with your Maestro- or Creditcard.
A bigger amount in every bank with your passport.
The amount you get from an ATM differs too.
Bancolombia (I wouldn't use it).
Davivienda up to 400.000 COP (best exchange rate).
Servibanca up to 780.000 COP (only on Providencia).
Citibank up to 1.000 US$ in the moment ca. 2.300.000 COP (but not on San Andres and Providencia).
The best exchange rate is not always the best possibility. Don't forget that you might have to pay fees for each time using the ATM.