The paint bucket tool will fill an area with your chosen color. The default shortcut key for this is G. View the video below for more or scroll on for the written guide:

On the right, underneath your color wheel, you’ll find a range of settings and behaviors for your paint bucket.

This setting determines which layer’s contents will inform your fill.

Active: fill only according to the contents of your current layer

All: fill according the content of all layers

Above: fill area according the contents of the layer above your active layer

Below: fill area according the contents of the layer below your active layer

This setting determines what characteristics on your selected layer will inform the fill.

Color: this will create the fill on an area of the same color.

Opacity: this will create a fill on an area of the same opacity.

Smart: this will fill to the edges of a line or transparent area.

This will determine the strength of the fill tool and how much it will fill compared against other colours. A lower tolerance will mean that pixels will need to be more exact in colour to fill. Increasing the tolerance will increase the allowed difference in colour for pixels to be filled.

You might have to experiment to get the right tolerance level for your scenario. A lower tolerance level might also mean that you get more unchanged pixels towards the edge between two colours.

Contiguous: With this setting active, pixels of the same colour (based on tolerance level) need to be adjacent to each other. Deactivating this will mean that the fill will apply to all of the same colour across the layer, even if there are other colours or empty pixels between them.

Anti-alias: With this setting active, the fill will try to create smoother edges, or give the appearance of blurred edges, by having semi-transparent fill along the edges between colours. With it inactive, edges will be sharper and may appear jagged.

This setting gives you some options for the behavior of the Fill tool.

Fill: standard fill based on either color or opacity

Fill rest: an content outside of the shape of color you choose will fill

Clear: instead of filling an area with color, it will erase that area instead.

Clear rest: erase the contents outside a specified area

The opacity slider will control how transparent or opaque the fill action will be. Lower percentage will be more transparent.