Can you think an answer, how a reporter enzyme can be used

Can you think an answer, how a reporter enzyme can be used to monitor transformation of host cells by foreign DNA in addition to a selectable marker?

A reporter enzyme can be used to monitor transformation of host cells by keeping a track of its receptor gene. Examples of reporter enzymes are p-galactosidase or alkaline phosphatase. They have particular characteristics that allow visual or spectrophotometric detection of the activity of their corresponding genes - If host cells are transformed by the uptake of foreign DNA, that will cause insertional inactivation of the enzyme gene. The gene product will not expressed and the colonies of the host cells will appear colourless in the presence of a chromogenic