Apple Color 1.5 - Manual (Page 298)

Difference
The pixels from the image that’s connected to Source 1 are subtracted from the pixels
from the image that’s connected to Source 2. Black pixels have a value of 0, so any color
minus black results in no change to the Source 1 image. The order in which the inputs
are connected matters.
This node is useful for darkening the Source 1 image based on the brightness of the
Source 2 image.
Multiply
The pixels from each input image are multiplied together. White pixels have a value of
1, so white multiplied with any other color results in no change to the other image.
However, when black (0) is multiplied with any other color, the result is black.
When multiplying two images, the darkest parts of the images remain unaffected, while
the lightest parts of the image are the most affected. This is useful for tinting operations,
as seen previously, as well as for operations where you want to combine the darkest
portions of two images.
Creating Layered Effects Using Mattes
An extremely important method of creating layered effects involves using a grayscale
matte to control where in an image two inputs are added together. The Alpha Blend
node has three inputs that work together to create exactly this effect.
This node blends the Source 2 input to the Source 1 input in all the areas where the
Source 3 Alpha input image is white. Where the Alpha input image is black, only the
Source 1 input is shown.
298 Chapter 11 The Color FX Room