To fill an area with color in Photoshop, you can use the Paint Bucket tool, which is located under the Gradient tool in the toolbar. You can select it by right-clicking on the Gradient tool icon or by pressing G on your keyboard. Then, you can choose a foreground color from the color picker or by using the Eyedropper tool. Next, you can click on the area that you want to fill with the foreground color. You can also adjust the settings and options of the Paint Bucket tool in the options bar, such as mode, opacity, tolerance, anti-alias, contiguous, and all layers.