The normal three skilling stamp printed in Sweden is colored
green while the eight skilling stamp was printed in yellow.
However, due to an unknown error (but most likely the three
skilling plate was accidently replaced by an eight skilling
one), the three skilling stamp was printed in yellow, creating
the Treskilling Yellow Stamp.
The only known copy of the Treskilling Yellow was discovered by a
schoolboy named Georg Wilhelm Baeckman in 1886 while going through
his grandparents' attic.
To this date, this is the only copy known in existence and is worth
about $80 billion/pound by weight - one of the most expensive
objects in the world!