I recently updated Particle Forge to v 2.0, and one of those new features was to plug the fresnel in alpha, giving you more control over color.  You no longer have to use Black To Alpha with fresnel since it calculates the alpha now, and to showcase this I made a new particle that gets darker while also moving on the alpha channel. 

Before if I wanted a dark particle, I'd have to make a light one and then edit it in GIMP or Krita.  Now I have eliminated that seocnd step