The development of plastic card printers has led to the widespread use of identity documents printed on plastic cards, such as ID cards, driving licenses, and access key cards. This paper presents a new security feature based on a technique for embedding a personalized microstructure into an image. This microstructure takes the form of a pattern embedded into the original photograph as a succession of balanced chromatic shifts. The amplitude of these shifts may be tuned so as to make the pattern fully apparent or just noticeable under normal viewing conditions. Since the chromatic shifts cancel each other out in any macroscopic portion of the image, the global appearance of the protected image remains intact. The embedded microstructure may be adapted to each instance of the protected identity document. For example, it can repeat textual information already present elsewhere on the document, or it can include a code derived from data specific to the document holder. Furthermore, this information may be made fully readable without requiring special revealing means. Such identity documents exhibit an intrinsic resistance against imitation, tampering and substitution.