How do I change the bucket fill colour in Gimp Image Editor?
Even though it is late I want to add a particular case:
Reopen your Toolbox window
If you need to reopen your toolbox, use the key combination
Ctrl+B or navigate through (GIMP version 2.8.14):
Windows > Toolbox
In this toolbar you can change the color by just clicking on the foreground or background color as indicated by @OrangeTux and @Warren Hill.