Always on the lookout for color varieties, I came across some variation in the Denmark 1977 Water Lily’s stamp from the Protection of the Environment set.
The color difference appears seems to show missing or underinked green coloring on the lily’s as well as the posthorn.
My copy of the correct color stamp is a bit damaged unfortunately, but I have 2 copies of the stamp with the color difference. This is likely a common variety.
Here is the full stamp with the color difference. Note the color of the Lily’s as well as the shading around the flowers and the posthorn.
Here is a closeup comparison
Not sure what caused this difference in color, but it is an interesting variety.