problem with set color to alpha

[Blighty]

(02-02-2019, 01:35 AM)rickk Wrote: SO, I painstakingly draw a white area  all around the perimeter of the desired target image

It would be better to use the Eraser tool. Select a suitable brush (eg round, hardness 100) and erase the unwanted area.

There are also Gimp's selection tools. Use one of these to select an area, then do Edit > Clear. And when you get to know Gimp a bit better there are Layer Masks.

Quote:Almost seems like the "color to alpha" function isn't discriminating properly, and is perhaps shifting all greens to alpha?

Yes, that is the way "color to alpha" works. Use one of the selection tools to select an area, then use "color to alpha" on that area. The parts of the image outside of the selected area are not affected.