Interesting. It is difficult to guess what is the exact process. For example, there are some programs that try to mimic particles and inclusive moving particles. For example this: http://www.escapemotions.com/experiments/fluid_painter/
I will try to do some experiments later but one option is to start in Photoshop with a simple black and white noise and start smearing it in the desired direction.
I think you can try in a low resolution and resample it later.
I would use separated layers and you can change the color of the layer and not contaminate the adjacent colors.
Another option would be trying with a painting program, like Corel Painter or the free one MyPaint, or Dogwaffle.
Note that this is not a definitive answer, but does answer:
tips in order to start moving on
One first example. This is a test on Dogwaffle Artist 9 using some particle brushes directly:
One more place to explore is using a pressure sensitive tablet.
In photoshop you can change the dynamics of any brush, sometimes the pressure changes opacity or color. In this case, you want to change size.
Source : Link , Question Author : Community , Answer Author : Cai