In the Layer Options section (see Layer options) we explained some of the various options available in the "Layers" dialog. One of the most useful is the Mode option, which determines how colors on the active layer will blend or combine with colors on the layers below. The effect of a Blend mode is usually obtained in conjunction with a setting for the Opacity of the layer.
There are several options for blending. The list of these is the same as for Brush modes (see Brush modes). There are, though, important differences:
brush modes concern blending between colors on one layer, whereas blend modes concern blending between different layers;
brush modes can be limited to a selection on a layer, whereas blend modes work on the whole of a layer;
a brush mode will only take effect once you start to paint with it, leaving previously applied brushstrokes untouched unless you paint over them, whereas a blend mode is applied immediately it is selected to the whole of the layer and will continue to be applied as long as it is selected;
brush modes are permanent, unless you use the Undo option in the Edit menu, whereas blend modes can be changed or cancelled by selecting another blend mode.
The usefulness of blending can be gauged from the following notes.
The image window can contain many layers in a definite order. Each layer can have its own image but all the layers share the same "space", namely the standard image window. However, when it comes to displaying or printing an image that is comprised of a series of layers, it is obviously impossible to display all the separate layer images. In other words, only one image can actually be displayed. Blending is a way of determining the final color values for an image you wish to display or print, when that image consists of more than one layer. Note that the Blend modes do not effect a permanent change and can be cancelled at any time by selecting Normal in Mode and setting the Opacity at 100%.
For some examples of blending between layers, see Blending between layers: examples.