For existing image files and a plain colour background you can use the batch plugin BIMP https://alessandrofrancesconi.it/projects/bimp/
Set the FG colour. Start BIMP Add the image files and use 'other gimp procedure'.
Search for fill, choose gimp-bucket-fill, settings as below. Set a output location and run it.
A bit more difficult to add a background image. You can open all your images as layers, and merge another image under each layer.
There are plugins to do this: http://sourceforge.net/projects/gimp-too...s/scripts/ look for ofn-interleave-layers.zip (dated 2020-04-25) Read the documentation that comes in the zip. Then you need to re-export all the layers back to single images. Same place look for ofn-export-layers.zip (2018-02-22) There are other scripts/ plugins that do similar.
Set the FG colour. Start BIMP Add the image files and use 'other gimp procedure'.
Search for fill, choose gimp-bucket-fill, settings as below. Set a output location and run it.
A bit more difficult to add a background image. You can open all your images as layers, and merge another image under each layer.
There are plugins to do this: http://sourceforge.net/projects/gimp-too...s/scripts/ look for ofn-interleave-layers.zip (dated 2020-04-25) Read the documentation that comes in the zip. Then you need to re-export all the layers back to single images. Same place look for ofn-export-layers.zip (2018-02-22) There are other scripts/ plugins that do similar.