der Kollege
colleague
A person you work with, especially in the same company or in the same department.
Mein Kollege aus dem Marketing hat mir bei dem Projekt geholfen.
My colleague from marketing helped me with the project.

colleague
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colleague
A person you work with, especially in the same company or in the same department.
Mein Kollege aus dem Marketing hat mir bei dem Projekt geholfen.
My colleague from marketing helped me with the project.

'Kollege' (colleague) is an N-declension noun. This means it takes the ending '-n' or '-en' in all cases except the nominative singular. Learners often make this mistake.
Ich frage meinen Kollegen, ob er Zeit hat.
I'm asking my colleague whether he has time.
In a professional context, 'Kollege' (colleague) is often used with verbs like 'fragen' (to ask), 'helfen' (to help), or 'zusammenarbeiten' (to work together). Typical adjectives are 'nett' (nice), 'neu' (new), or 'ehemalig' (former).
Könntest du bitte einen Kollegen um Hilfe bitten?
Could you please ask a colleague for help?
In the meaning of 'Kumpel' (buddy), 'Kollege' is colloquial and used almost only as a direct form of address for a man. It expresses a relaxed, informal relationship.
Na Kollege, alles gut bei dir?
Hey buddy, everything good with you?
employee
'der Mitarbeiter' and 'der Kollege' can both refer to people at work. 'Mitarbeiter' describes more the role in a company or organization: the person is employed there or works on a project. 'Kollege' describes more the relationship to other working people: you work together with him or have the same profession. Learners can easily confuse the words because a colleague is often also an employee of the same company. As a practical guide: from the company's perspective, people often say 'Mitarbeiter'; from the perspective of a person on the team, people often say 'Kollege'.

Nach der Besprechung habe ich meinen Kollegen um Hilfe gebeten.
After the meeting, I asked my colleague for help.
Die Firma stellt im Sommer zehn neue Mitarbeiter ein.
The company is hiring ten new employees in the summer.