Trying to have particles mix with a light background color over life, to simulated fog over distance.
According to the manual, Cover of Life should 'Tint' colours over life, but it seems as though really to only option to to 'fade' colours to black over life, or rather reduce each color channels intensity.
Is making colours get darker really the only option here?
The alpha is determined by the particle image. If you're using one of the Blur images or Wisp images, their Alpha types are indicated as "Straight" (not premulitplied).
You can change/override the Alpha Type:
If you go to the Media tab (top middle of Layers List) and select your particle images, then look in the Media tab in the inspector, the first parameter is Alpha Type. Set the Alpha Type to Premultiplied - Black and you'll get rid of the black edges.
Telling Motion the "truth" helps it handle the images more accurately. (I figure that's why setting Premultiplied White seems to have the exact opposite effect than expected.)