After selecting the Bucket Fill tool in the toolbox, click the Pattern Fill radio button, and then click the thumbnail pattern that appears to select one of the available patterns.

For example, click the "Rectangle Select Tool" and click and drag on the image to apply a rectangle that displays a moving, broken outline. To fill this rectangle, click the "Bucket Fill Tool" icon.

Once youv'e made an outline, navigate to Select>From Path, or hold Shift+V to activate your selection, then use the Bucket Fill Tool or Paintbrush to completely fill in the selection with white.

Duplicate the layer (Layer>Duplicate Layer) one time for each color you want to use, then use the Paintbrush or Bucket Fill tool to make the image a different color in each layer.