Well, the background color should also extend to the area of the denomination, which is gray instead. Obviously, the background color is wrong and may be changed by light or chemistry, but why gray behind the denomination?
The impact of color loss through light exposure is influenced by thing "laying on top". Which can explain why there is variation from within a single color. Also when thinking of color, remember your primary colors and mixing: Yellow + Blue make green, Red and blue make Purple, Yellow + Red make Orange, and then you go from there. So to go from Purple to Green is not a stretch, because you have a red pigment that is lost to light, the "blue" if not "true blue" would be a mix of some blue and yellow potentially which give you a greenish hint.
What you don't find is blue turning orange or yellow turning purple. Those are the results of chemical alterations. (Though chemical alterations can also just remove a primary). The key here also is to remember that color changed, is color changed, no matter what the source, whether natural, artificial, intentional or unintentional... it's then an "altered" stamp by definition.
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Also when thinking of color, remember your primary colors and mixing: Yellow + Blue make green, Red and blue make Purple, Yellow + Red make Orange, and then you go from there. So to go from Purple to Green is not a stretch, because you have a red pigment that is lost to light, the "blue" if not "true blue" would be a mix of some blue and yellow potentially which give you a greenish hint.
What you don't find is blue turning orange or yellow turning purple. Those are the results of chemical alterations. (Though chemical alterations can also just remove a primary). The key here also is to remember that color changed, is color changed, no matter what the source, whether natural, artificial, intentional or unintentional... it's then an "altered" stamp by definition.