Your color shift appears quite minor and based from what I see on the internet, it is probably within acceptable range as being of no additional value than that of any typical stamp from that issue.
Interestingly,
ebay does carry listings for "major" color shifts (both left and right) which are said to not be listed in either Scott or Unitrade, but are listed in the Darnell Catalog as #443a with a premium value of around $30. However, I don't think your example would fall into that category, as the color shift on your example is not that severe.