"Kollege" is meant to be masculine here. There is a feminine version "Kollegin" as well. This is one of a subclass of masculine nouns where most inflections end with "-n". So not only is the plural "Kollegen", but the accusative, dative, and genitive, both singular and plural, are all "Kollegen". There are a number of these nouns; they are mostly people, "Bauer", "Junge", "Kunde", "Student", ... and certain animals, "Hase", "Bär", ... . In this case it's the accusative so "Kollegen" is correct. The singular and plural both have the "-n" ending, so how do we know that it's supposed to be masculine and not plural? That's where the "ihren" determiner comes in. It also has the masculine accusative ending, so "ihren" for masculine rather than "ihre" for plural. Figuring out the correct combination of gender and case from German's rather complex system of inflections can take some getting used to.
German has four cases altogether, nominative, accusative, dative and genitive. (This is the order English speakers learn them, native German speakers use a different order.) Nearly all words in German, excluding verbs, are inflected according to case among other factors such as gender and number. The accusative and dative case of singular nouns is usually the same as the nominative case, which is the dictionary form. But there are exceptions and "Kollege" is one of them. (Some grammars call them "weak" nouns, as noted in the comments.) You may want to check other resources for a summary of German cases and inflections. If you're starting out then try Grimm Grammar. You might also try Bruce Duncan's A Review of German Grammar for a slighlty more advanced overview. There are other resources listed in the FAQ for this site as well.