label 10 is exposed to excessive heat in this example the color of the opaque printing 20 would be white the visibility of the hidden opaque printing 20 in fig1 is for illustration only where again the opaque printing 20 would be substantially invisible in actual practice against the background of the label 10 the hidden printing 20 could be added to the label 10 by inkjet printing for example when the label 10 is exposed to excessive heat the background color of the label 10 turns dark as shown in fig2 exposing the opaque printing 20 and rendering it visible e g to reveal a message the illustrated label 10 in fig1 and 2 could be used for example for safe guarding medication in pill bottles from excessive thermal exposure the safe guard would be an integral part of the label on the bottle in this embodiment the opaque printing 20 is a warning message preferably pre printed on a white direct thermal label 10 using opaque white ink the white on white printing is initially invisible when the label 10 is exposed to excessive temperature the entire label images or turns dark the initially invisible white printing 20 becomes visible this is shown in fig1 and 2 the activation temperature when the label 10 turns dark can be selected based on requirements for safe guarding particular medication.