An example of what you want to fill would be nice.
Other than that, with the bucket tool you got plenty options the way you want to fill
> more about it https://docs.gimp.org/en/gimp-tool-bucket-fill.html all options explained.
Colors > Colorize... could be an alternative inside a rough selection depending your image and options given to colorize
Also Colors > Map > Color exchange... could be the one you need
But an excerpt of your image is needed for us to see what you need, or the fastest way to do it
Other than that, with the bucket tool you got plenty options the way you want to fill
> more about it https://docs.gimp.org/en/gimp-tool-bucket-fill.html all options explained.
Colors > Colorize... could be an alternative inside a rough selection depending your image and options given to colorize
Also Colors > Map > Color exchange... could be the one you need
But an excerpt of your image is needed for us to see what you need, or the fastest way to do it
Patrice