Paintbrush
Use the Paintbrush tool [B] to paint color onto a picture. This tool’s most basic option is its brush color. To change it, click the colored rectangle (this calls up a color mixing window) or use the eyedropper to choose a color from the picture itself. The other settings, like the Radius, Opacity, Density, Blur, and Spacing work the same as they do for the other tools, e.g. the Clone Stamp. Use the Mode setting to set how the color painted on by the brush is blended into the original picture.
Fill
Use the Fill tool [G] to fill an area with a color of your choice. The program sets the fill area based on its similarity to the spot you click. It judges similarity based on the Mode setting (RBG, Brightness, or Hue) and the chosen Tolerance. The Continuous option determines whether the fill will “spill” out into all similarly-colored areas in the picture, or just one continuous area. To use the tool, click on a spot within the area that you want to fill.
Gradient
Use the Gradient tool [Shift+B] to add a multicolored gradient to the picture. The gradient is defined by two points. To create the gradient, left-click in the picture to set the first point, and then drag to set the second point. Click Gradient in the Side Panel to adjust or add the gradient’s colors and transparency. Scale sets how sharp the gradient is; Offset shifts the start and end of the gradient relative to the two points set. Three Types for gradients are available—linear, radial, and angular. There are also Opacity and Mode settings that control how the gradient is blended with the picture.
Eraser
Use the Eraser [Y] to erase part of the picture. The Radius, Opacity, Density, Blur, and Spacing settings work the same for this tool as for other tools. Erased parts of a picture are transparent.