Paint.NET's Paint Bucket tool is a quick way to add and replace colors within a design or image. In some applications, Paint Bucket tools will fill a complete layer or just an area within a selection, but the Paint Bucket tool in Paint.NET is more sophisticated than that. It will only change the color of pixels that are the same color as, or similar to, the pixel that the Paint Bucket tool was clicked on.
For example if you had a black square in the middle of a white page, if you clicked on the black square, only that would have its color changed and the white page would remain the same. You could further restrict the area that had its color changed by making a selection before using the tool.
When active, the Paint Bucket tool has a number options displayed in the Tool Options bar that alter the way that the tool works.
Flood Mode
There are two settings available for this option. The first, Contiguous, means that the new color will only be applied to pixels that form part of the same area of the image as the pixel that was clicked. For example, if you had two separate objects of the same color in a page that were separated by an area of a different color, clicking on one object will only change the color of that object.
If you change the mode to Global, pixels no longer have to be connected to the pixel that was clicked, they just need to be the same color. In this case, if you clicked one of the two same colored objects, both of them would change color.
Fill
This drop down offers a range of options that allow you to apply the new color as a pattern rather than as a solid color. This may be useful when producing designs, but will probably have less value if you're working on a photo.
Tolerance
This control is actually a slider and you can adjust the setting by clicking within it to change the value. When adjusted, the number value changes and the size of the blue bar changes also in relation to the number.
If the Tolerance is set to zero, pixels must be the same color as the pixel that is clicked on in order to have their color changed. As you raise the Tolerance setting, more colors that are similar to the clicked pixel, but not the same, will be included in the area that has the color change applied.
If you're working on flat colors, then leaving this slider set to 50%, which is the default, is probably the best setting. It is worth noting that if the Tolerance is set high enough, it is possible that every pixel in a layer will have the new color applied to it, regardless of their original color.
Anti-Aliasing
When enabled, the edges of of the area where color has been applied should appear smoother and blend more neatly with surrounding pixels.
Blending
The effect of the two settings here is really only visible when semi-transparent colors are being applied. When set to Normal Blending, the original color of each pixel may have a bearing on the final color of the pixel, so if a semi-transparent color is being applied, it will blend with the original pixel color to make a new color. When set to Overwrite, the original pixel color is erased and the new color is applied, so if you are using a semi-transparent color, a checkerboard pattern will become visible through the newly transparent pixels.
The various options that are available with the Paint Bucket tool mean it is a powerful and flexible tool that can be used to apply color and patterns in a variety of different ways.