Standard Fill
The Standard Fill Tool will flood an enclosed area with the color and method (solid color, gradient, image tile, pattern, etc.) chosen from the Fill Selector and Color Palette. The left mouse button will fill using the color on the left side of the Color Box. The right mouse button will fill using the color on the right side of the Color Box.
To use this tool, click the mouse button once in the middle of the area in your picture that you want to fill. The pixel you click serves as the seed point where the fill process will start. From there, pigment will spill onto all adjacent pixels that match the seed pixel. This continues until another color or the edge of the picture is reached.
See Fill Tools for more details on using this tool.