Maybe you wonder about those Pantone-like stamps. I sent some more:
They belong to a campaign against racial discrimination. In May 2021, Correos launched Equality Stamps to reflect "an unjust reality that should not exist, in an effort to raise awareness of diversity, inclusion and equal rights". (See the video here).
The idea behind is that you must used more black stamps than "white" ones. But what the users read was: the darker the colour, the smaller the face value. The campaign received a lot of criticism, and the stamps were withdrawn from the market almost immediately.