Use Flood Fill 
to fill an area of the picture with the selected color. The pointer changes to the paint bucket 
. The area to be filled is defined by other colors surrounding it or the edges of the work area. Click to fill an area using the selected color. Flood fill will not flow across gaps between separate areas of the same color. Either fill each area separately, or use other drawing tools to 'patch up' a shape to make it a continuous area of the same color.
Shortcut
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Paint Window Control Panel, Tools:
Flood Fill an Area
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Select the desired color in the Color panel. Use Pick Color
if you want to select colors from the picture.
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Click the Fill icon
and the pointer changes to the paint bucket
. -
Change the color sensitivity of Flood Fill with Tolerance
, by adjusting the slider.
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Click on a color within your picture. Adjacent areas in the picture with a similar color to the area that you clicked are filled with the foreground color (or the background color, if you right-click).
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Flood Fill is enabled until another tool is activated, or you right-click to deselect it.