In the context of a magazine, "colaborador" would never be translated as "collaborator", so the example "The collaborator of the magazine" is erroneous. In this context, "contributor[ is always used, and the accompanying preposition is always "to": "The contributor to the magazine..."
Dictionary entry: colaborador
Thanks for the input, Kite Man. The entry will be changed as you suggest, after the next dictionary update within a month or two.