The blue-white technique is only a screening procedure; it is not a selection technique. The lacZ gene in the vector may sometimes be non-functional and may not produce β-galactosidase. The resulting colony will not be recombinant but will appear white. Even if a small sequence of foreign DNA may be inserted into MCS and change the reading frame of lacZ gene. This results in false positive white colonies. Small inserts within the reading frame of lacZ may produce ambiguous light blue colonies as β-galactosidase is only partially inactivate - See more at: http://www.sigmaaldrich.com/technical-documents/articles/biology/blue-white-screening.html#sthash.9ziOJCSA.dpuf