Royal National Park NSW

Hi to all

This stamp is the Royal National Park New South Wales is was issued in 1979 in two of the stamps you can definitely see missing colours. One the park is lush and green and the other looks drought stricken. They have not been left out in the sunlight or exposed to light.
Any ideas.
Regards
Horamkhet

There's always a difficulty with missing colours in loose used stamps: you haven't tinkered with the stamps in any way, but you can't know the entire history of the stamp from the time it was bought in the original post office until it reached you. Who knows what happened to it in the time before it came into your hands?

More specifically with these stamps, there seems to be a definite gradation in the shades of the green across the four stamps. The most likely explanations are that there were two printers used for this issue, and/or that the variation in shade was within accepted tolerances, and/or that they have indeed been exposed to the Sun at some point in their histories.