Instead of tetracycline gene, pUC vectors have a lacZ gene coding for J3-galactosidase enzyme. This gene helps in identification of plasmid pUC with insert and without insert. If we plate these colonies on a medium containing J3-galactosidase indicator, colonies with the pUC plasmid will change colour.
When bacteria are plated on the medium with X-gal and IPTG, the 13galactosidase enzyme will break X-gal (colourless) to release galactose and X- (indigo dye), which stains the bacterial colony blue.
On the other hand if we have interrupted the plasmids by placing a gene of interest is the multiple cloning site, the gene becomes inactivated. Hence the bacteria fails to be stained and remains white.
Thus, it is a simple matter to pick the colonies with inserts as they are the white ones and bacteria without insert are blue. Notice that in relation to cloning an insert in pBR 322, it is easier to clone in pUC vectors as we have to just look for a clone which is growing on the medium with white in the presence of X-gal.
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