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:
Normal The color is applied in full, that is to say, the existing base color is totally over-painted.
Dissolve A color applied with a less than 100% opaque brush is converted - "dissolved" - to 100% opaque randomly scattered pixels. The effect is of a rough brushstroke rather than an evenly transparent one.
Multipy The brightness values of the base and applied colors can be combined to create a darker tone. This is done by multiplying the two brightness values when they are expressed as fractions. For example, white has a brightness value of 255 (the maximum) and is represented as 255/255 or 1; black has a brightness value of 0 (the minimum) and is represented as 0/255 or 0. All other brightness values are represented as fractions between 0 and 1 e.g. 150 is represented as 150/255; 100 as 100/255. Therefore, using Multiply we get the result (100 * 150) / (255 * 255), which works out at 59/255. You can immediately see that the resultant brightness will always be lower than (or equal to) the brightness of either one of the multiplied fractions. This intuitively fits in with our experience of placing one transparent sheet on top of another, where the result is always darker.
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).
Screen This mode has the opposite effect from Multiply, combining the base and applied colors to create a lighter tone.
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: