How far out does a colour have to be, before it is considered an error?
The reason I ask is that I have come across two stamps in my collection that have different colour backgrounds, but were only released in one colour. One is orange and the other is yellow. I don't think one has faded as the other colours all look normal.
Combined picture, to emphasise to difference...
The stamp is SG-661 issued in 1978 as part of an aviator series.
Any info in your Scott catalogs?