why not use the particular layer as an luminence matte and then use a gradient overlay to provide the color. That would achieve the effect you are showing in the still image. Basically set up you particle movement using particular and set the particles to a mid gray with screen as the transfer mode. make another layer and apply your gradient. If you want the rainbow look, apply a linear gradient from right to left from black to white and then apply colorama and tweak you multicolored gradient. Then, use the transfer modes or track mattes to apply the gradient color to the particles.