This is as far as my understanding of gene cloning goes. Let's say that the plasimd (vector) in the bacteria contains an ampicillin resistence gene. Through restriction enzymes, the ampicillin resistance gene is cut through and foreign DNA is inserted. This creates a recombinant DNA molecule.
Once many bacteria are introduced to a medium that contains ampicillin, only ones that have no recombinant DNA and functional ampicillin resistance genes will survive and those with the recombinant DNA won't. Through the addition of some chemicals, the surviving bacteria will turn blue and those with the recombinant DNA will turn white. Thus, bacteria with recombinant DNA can be taken and grown in their own culture, the recombinant DNA can be isolated, and research goes on.
However, if all the bacteria with recombinant DNA have been killed because they have no functional ampicillin resistance genes, how can they be 'grown' in their own colony? They're all already dead, aren't they?