Brush modes

You can set a brush mode for several of the tools, for example, Pencil, Paintbrush, Duplicate, Fill and Gradient Fill. The brush modes determine how an applied brush stroke affects the colors already in an image. Essentially, this can be understood in terms of the existing or base color of a pixel, the color that will be applied by the brushstroke, and the color that results from the combination of the base and applied colors, as determined by the selected brush mode. For instance, if you paint a blue line over a patch of red in an image, do you want the red to be completely replaced by the blue, or somehow blended with it? The brush modes allow you to decide how and to what extent the applied color will affect what is already there.

The main options, which you can select in the Mode box in the "Brushes" dialog, are:

Note that the above means that if the applied color is white, the resultant color is the same as the base color. If the base color is white, the resultant color is the same as the applied color. If either the base or the applied color is black, the resultant color is black. If the base color is transparent, the result of applying any color is black (since a transparent layer counts as 0, that is the same as black).

These are some of the more important options but there are many others that it is worth experimenting with:

Behind, Overlay, Difference, Addition, Subtraction, Darken Only, Lighten Only, Hue, Saturation, Color, Luminosity, Divide, Color Burn, Color Dodge, Hard Light, Soft Light

See:

Brushes