Reasons to think the entire country of Colombia is a tropical paradise: Colombia is in South America, Cartagena is on the Caribbean, and everyone knows there is plenty of jungle inside its borders.
But Bogota and Medellin are not exactly resort towns. First of all, they're located at a high elevation. In the capital, Bogota, the temperature typically stays within 40°F to 65°F. In Medellin, city of eternal spring, the temperature stays within 60°F to 80°F. So if you are planning to spend a week in Colombia and are wondering what to wear in Medellin and/or Bogota, pack as if it's early spring in the Northern hemisphere. Note that you can pack slightly lighter for Medellin if you're not making a stop in Bogota (which is entirely possible, it's not exactly a tourism magnet).