Bologna sack coach day before season opener
"He was too sceptical about the team's chances in general," club president Sergio Porcedda, who took over the club in the close season, told a news conference.
"The decision has been taken solely by me and I'm very sorry for him. I slept with a lump in my throat."
Colomba led a modest Bologna side to fourth-bottom in Serie A last term as they survived by seven points.
Much-travelled coach Mario Beretta and Alberto Malesani, whose Siena side were relegated from the top flight last term, are among the favourites to replace Colomba with Porcedda saying a decision would be made by Wednesday.
Youth team coach Paolo Magnani takes charge for Monday's Inter game.