Inspired by Flame Painter, I noticed that Particle Brush seem to be lacking in comparison with Flame Painter, and you have to go through all the trouble adding gradient map filter mask/layer, and clone your painting layer, and apply destination in just to get similar effect. So, my suggestion is to support adding randomization of colors via alpha information, and gradient map color utilizing alpha information, and allowing rainbow stroke using stroke line as reference. 

To clarify:

For randomization of colors via alpha information, it shall use shifting color with random values,using alpha information, and if you zoom in closely, the colors are smooth during the transition.

And then, gradient map via alpha information. That would be hard to emulate, but I think it use alpha information as the base for generating colors on the particle brush.

And finally, there should be an option to allow rainbow stroke using your stroke line as information as to values of color going by rainbow path.

These are extension of particle engine I believe artists would find useful.