You can read more about how the eff has determined that 'all major printer manufacturers have a secret agreement with governments to implement tracking'. There's no link to the specific agreements though since they are secret. It's just known that printers add colored docs to track documents and it's also known that this is required by various governments throughout the world.

https://www.eff.org/pages/list-printers-which-do-or-do-not-d...