Select the Paint Bucket tool from the Toolbar. It shares the button with the Gradient tool, so the tool that was last used will be the one that is visible.

Select whether you want to fill with Foreground Color, or with a pattern. We'll start with Foreground Color.

Select the Foreground Color. Click in the Foreground Color if you wish to change it. That will open the Color Picker.

For a detailed guide on both of these, see Foreground and Background Color and Color Picker.

In the Color Picker, click in the Color Slider (highlighted in green) to pick a color. Then click in the Picked Color (highlighted in pink) to select a tone.

With the Color Picker open, you can also click outside the Color Picker, onto an open image, to select a color direct from that image.

Click OK to close the Color Picker.

I have selected pink as the Foreground Color.

I have a new, empty image. Just click with the Paint Bucket tool onto the image.

The image is filled with the Foreground Color. The cursor (highlighted in green), shows where I clicked. In this image, with pure white, it does not matter where I click. In other images with more complex content, it does matter where you click, as detailed the the steps below.

When you click, the area that is filled will depend on Tolerance and Contiguous settings, as well as the layer (or layers) image content. The higher the Tolerance, the more pixels will be filled. If Contiguous is turned on, only pixels that are touching other selected pixels, will be filled. If Contiguous is turned off, all pixels that are within the Tolerance will be filled.

Let's return to an empty document and fill with a pattern.

Select Pattern, instead of Foreground.

Now the Pattern button is available. Click on it to select a pattern. I have selected the first of the Grass patterns.

One click and the layer is filled with the pattern.

The whole layer is filled because it was originally filled with a plain color, without any variation in tone or color. This means that all pixels were within the default Tolerance of 32. Whether Contiguous was turned on or off would not make a difference, because all the pixels are exactly the same color and are touching each other anyway.

Now the layer is filled with multiple tones and colors. Let's see what happens if we try to fill with Foreground Color.

All the green parts of the layer have been filled with the pink Foreground Color.

This is because the pixels were within the Tolerance of 32 and also Contiguous was turned OFF. In this image it makes a big difference if Contiguous is turned ON, as the next steps will illustrate.

Contiguous has now been turned on. That means that only pixels that are within the Tolerance of 32 (32 tones lighter and darker than were I clicked) AND are touching other selected pixels.

At first glance it looks like nothing has happened

Something did happen, but we need to zoom in heavily to see it. See the next step.

Very few pixels were filled with the Foreground Color of pink. This is because of the Contiguous setting is turned on. Only pixels that are touching other pixels that also are within the Tolerance, will be filled.

I undid the previous step and changed Tolerance to 80 (from its default value of 32). So now pixels will be filled that are within 80 tones lighter and 80 tones darker than the spot I clicked.

Contiguous is still turned on, so only pixels that are touching other selected pixels are filled.

The next step shows a close-up.

This is a close-up of the previous step.

I have undone the last few steps. Let's try again, but this time select the leaves, rather than the grass.

This is the original pattern before we try to Fill with Foreground Color.

This time I have clicked in two areas, indicated by the green circles.

Tolerance is 80 and Contiguous is turned OFF.

The amount of leaves that was filled, is quite different. See the next step for a close-up, to see clearly why this happened.

This is a close-up of the previous step.

I have clicked in two places, but importantly, Contiguous is turned ON, so only pixels touching other selected pixels, are filled.

I only clicked once in each place, but the large area was selected because many leaves are touching each other.

The single leaf (near the bottom right corner) is not touching any other leaves.

I have set Tolerance back to its default setting of 32.

Contiguous is turned OFF, so all pixels matching the Tolerance setting will be filled.

See the next step for a close-up.

This is a close-up of the previous step.

The green circle shows where I clicked. At a Tolerance of 32, not all the leaves are selected, even though Contiguous is turned off.

Tolerance has been increased to 80, as in earlier steps.

I have clicked in the same place as the previous step, on a leaf. Contiguous is turned OFF, so all the leaves are filled.

Also, some of the grass has been filled too, as it was within the Tolerance of 80.

See the next step for a close-up.

This is a close-up of the previous step.

Virtually all of the leaves are selected, with the exception of a few very small areas. Some of the grass has been selected too, because of the high Tolerance setting.

If you have multiple layers, the All Layers setting will make a difference to which areas are filled.

In this example, I've added a new empty layer above the layer containing the grass and leaves.

The Paint Bucket tool has been used with the same pink Foreground Color, as in earlier steps.

Tolerance is 80, as above and Contiguous is OFF.

Although clicking on an empty layer, because the All Layers is selected, the Paint Bucket takes into account the content of all layers and only fills the empty layer as shown.

This is the result of exactly the same Tolerance and Contiguous settings as the previous step.

All Layers have been turned off, so the Paint Bucket is only looking at the content of the empty layer.