The left stamp is St. Vincent #2240G(1Jan1996), priced at 55¢ in 2018 Scott. In Michel, G 3371.
Neither list color variation or any souvenir sheets for this issue.
It is either a color changeling or a yellow color missing error (yellow + blue = green). Most likely the former. I don't know what method of lithography was used.
To check for bleached (sun-faded) yellow, look carefully near the black ink cancels. The black ink helps to mask against bleaching effects. If you see any sort of slightly greenish color along the blurred edges of the cancel (easiest to look around the SW section of the outer circle), then color changeling. You will need a good magnifier.
It's been my experience that EFO's of this type are color changelings.
We are Happy to have you here!! New members, we would love to get to know you. Please feel free to introduce yourself HERE
Otherwise - Jump right on into the conversations!! We look forward to your participation.
If you are not a member yet, register today!! It's all free and we want to hear from you!!
Please share our site on social media networks. Thanks!!!