Outside upper-class circles in the bigger cities, Colombian's don't speak much English at all. While those working in the tourism industry often have a working knowledge of the language, if you really want to get out and explore, some basic Spanish is a great help.
Universities and language schools in the larger cities run Spanish-language courses. It is generally cheaper and better value to arrange a private one-on-one tutor. Popular backpacker hotels are the best places to ask about independent teachers. Enrolling in a university course is useful if you want to extend your stay beyond the six months a tourist visa permits you.