What feature indicates that codes are valid?

The feature that indicates codes are valid is the presence of a watermark. Watermarks are designed to provide an extra layer of security and authenticity to documents or codes. They are often difficult to replicate and can be used as a verification tool to ensure that a code is genuine and not forged. This is particularly relevant in contexts where validation and security are essential, such as in financial transactions, identification cards, or official documentation.

In contrast, while printed signatures, serial numbers, and color coding may offer some form of identification or categorization, they do not serve the same purpose as a watermark in terms of enhancing security and validating authenticity. A printed signature can be forged, a serial number can be replicated, and color coding can easily be imitated. Therefore, the watermark stands out as a more robust feature for confirming the validity of codes.