Apple Motion 2 Tutorials - Manual (Page 173)

Tutorial 4 Working With Particles 173
3 In the Blending controls, choose Add from the Blend Mode pop-up menu.
Note: You can also Control-click the particles layer in the Layers tab and choose Blend
Mode > Add from the shortcut menu.
Now, the color values of the particle system are added to the color values of the
background instead of replacing them, so the background shows through.
4 Save your project (press Command+S).