From AdaptableGIMP Wiki

Create a black and white image where some parts of the image still have color.

Commands
Fuzzy SelectFuzzy Select Feather (dialog)Feather (dialog) InvertInvert Channel Mixer (dialog)Channel Mixer (dialog) Free SelectFree Select Rectangle SelectRectangle Select

Description

Step 1. Open a color image.

Step 2. Use the Fuzzy SelectFuzzy Select command (or other selection tools) to select the object whose color you want to preserve.

Step 3. Feather the selection using the Feather (dialog)Feather (dialog).

Step 4. InvertInvert the selection.

Step 5. Open the Channel Mixer (dialog)Channel Mixer (dialog) dialog.

Step 6. Check the "monochrome" box, and apply changes.