der Kollege — the colleague. The word came into German in the 16th century from Latin collega, which DWDS explains as Amts-, Standesgenosse, a fellow holder of an office or rank.
Herr Kollege, Frau Kollegin — the form of address among professionals. One of Duden’s examples is die Herren Kollegen von der Opposition, the gentlemen colleagues from the opposition.
kollegial — collegial. Duden: in keeping with a good relationship between colleagues. Das war nicht sehr kollegial von ihm. That was not very collegial of him.
mit kollegialem Gruß — with collegial greetings, a letter closing from one colleague to another