+ opt ⌥ = 
Paint Bucket tool in the Tools palette (or press shortcut N).
As you drag, the filled area grows to include the contiguous area that uses similar colors. You can control how much of the image is filled by dragging less or more.
Foreground color is used to fill.
Tool Options
In yhe Tool Options palette you can set a blending mode and opacity.
The Paint Bucket tool also has an option called "Anti-alias" to smooth the edges of the colored area.