"My friends did not like my colleague."
Translation:Mes amis n'ont pas aimé mon collègue.
If it were in imparfait it implies that your friends are in constant contact with, and continue to dislike your colleague whereas passé composé indicates they have met this person and disliked them - a past, specific, event. There is not enough context in the given sentence to justify imparfait.
And, no, aimer is not always in the imparfait.