Mapping all colors (except background) to a single color in GIMP
I have a spotty image. The image has maroon and black spots (with gradient) on a white background. I would like to convert every color (except white) to green color using GIMP.
As AndrewH mentioned, since the background is plain white, you can select it and invert the selection, then fill it with the color you want. Here's how.
Choose the "Select By Color" tool and click on the white background.
In the menu bar, go to "Select->Invert"(the shortcut is Ctrl+I).
In the menu bar, go to "Edit->Fill with FG Color" making sure the foreground color is the color you want. If you want to still keep the original image as it is in it's own layer, create a new layer, then fill the selection.