The official language of Colombia is Spanish. Some indigenous tribes in rural areas continue to speak their own languages, though almost all people from those tribes will be bilingual in their own language and Spanish.
If you've recently learned Spanish, its a relief to know that the Bogotá dialect is clear and easy to understand. The Spanish does vary, however, from Cartagena to Bogotá to Cali. Generally the Spanish on the coasts is spoken more rapidly, and Spanish from Medellín has its own idiosyncrasies. Note that in cities like Medellín and Cali, the dialect of Spanish is the voseo form. Meaning that instead of the second person familiar pronoun tú, vos is used instead. Though tú is also understood by everybody, vos is a more friendly voice while tú is reserved for intimate circles. The Spanish spoken along the Caribbean coast is similar to the dialects spoken in Cuba, Puerto Rico and the Dominican Republic.
Quite a few Colombians know at least a few basic phrases in English, since English is taught both at school as well as Hollywood movies tending to be in English with Spanish subtitles. For the most part however you should definitely invest in learning the basics of Spanish since you will encounter plenty of situations where no-one will speak any English.
Colombians from more affluent backgrounds will be more likely to have learned English, and the majority of high ranking professionals, executives and government workers in Colombia speak an acceptable level of English.