And yes, the code appears to be only printed in yellow.
[1] Timo Richter, Stephan Escher, Dagmar Schönfeld, and Thorsten Strufe. 2018. Forensic Analysis and Anonymisation of Printed Documents. In Proceedings of the 6th ACM Workshop on Information Hiding and Multimedia Security (IH&MMSec '18). Association for Computing Machinery, New York, NY, USA, 127–138. https://doi.org/10.1145/3206004.3206019
It is not “mainly HP printers”:
And yes, the code appears to be only printed in yellow.
[1] Timo Richter, Stephan Escher, Dagmar Schönfeld, and Thorsten Strufe. 2018. Forensic Analysis and Anonymisation of Printed Documents. In Proceedings of the 6th ACM Workshop on Information Hiding and Multimedia Security (IH&MMSec '18). Association for Computing Machinery, New York, NY, USA, 127–138. https://doi.org/10.1145/3206004.3206019