Why would you think it is a plug-in? Plug ins are just recorded actions made commercially.
Looks like they are just painting with a brush.
Curt Y wrote:
Plug ins are just recorded actions made commercially.
"Mr. President, that's not entiiiiirely accurate."
Plug-ins can do programatic things to image data - i.e., anything. Some Plug-ins can do amazing visual things, and I don't doubt that the above COULD have been done by a Plug-in. Bottom line is that a plug-in can do much more than an action.
It's possible, as Curt says, that the above was done with a brush, though notably brushing multiple colors at one time isn't trivial. It could be done by stroking a path, I suppose.