Two things.
If the mint stamp was not as exosed as the used copy then the colours would be faded.
But, on the right-hand stamp all of the green colour on the whole stamp is lightened or not as strong.
Could be a wash in hard water (has extra minerals in it) or a kind of soap wash from before has lessened the green. But, I think that this kind of semi-chemical washing would also lessen all the colours.
Wish I had one to compare. This is the kind of thing where you really need more than one copy of the stamp, mint and used, from different sources, to do some colout comparing.
I like it. Worth keeping. Probably would not be mentioned as a varietiy in the regular catalogues (constant only), but may be in a any-error-will-do kind of really interesting catalogue.
Check
ebay and Delcampe and other online sites, or dealers, taking yours with you.