He then served as the editor-in-chief of L'Unità, an Italian leftist newspaper, before leaving due to criticism he faced for his excessive independence from the party line.

He later cofounded and became a columnist for Il Fatto Quotidiano, from which he resigned during the Russian invasion of Ukraine because he said the paper had taken a pro-Russian stance and in protest of the hiring of Alessandro Orsini.

In the mid-1950s, he started working for the Italian national broadcasting corporation RAI, where he collaborated on the creation of cultural radio and television programs, documentaries and journalistic reportage.

In the early 1970s, Colombo taught "Theory and Techniques of Media" and "Radio and Television Language" in the newly founded Department of Art, Music and Entertainment at the University of Bologna.

On 16 July 2007, with an article published on L'Unità, Colombo announced his candidacy for the PD secretariat, focusing on strong criticism of Silvio Berlusconi.