Apple Motion 4 - Manual (Page 328)

Overlapping midrange color values are darkened based on the color of the background
image. In areas where the background is lighter than the foreground, the background
image is darkened. In areas where the background is darker than the foreground, the
colors are inverted.
The order of two objects affected by the Subtract blend mode is important.
Boxes object on top Monkey object on top
Darken
Darken emphasizes the darkest parts of each overlapping image. Whites in either image
allow the overlapping image to show through completely. Lighter midrange color values
become increasingly translucent in favor of the overlapping image, while darker midrange
color values below that threshold remain solid, retaining more detail.
The order of two objects affected by the Darken blend mode does not matter.
328 Chapter 7 Basic Compositing