How do I change the bucket fill colour in Gimp Image Editor?

Suppose We want to Fill an area in this image

The bucket tool uses the foreground colour so lets change it-- Click foreground

Set new colour

Click OK to accept

Foreground colour now changed select bucket tool

Use bucket tool -- Job Done

The black square in the highlighted area defines the foreground color. This color applies to all coloring tools, like the bucket

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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.