I just found something - haha sorry if you already know this !
But you know in, paint tool sai, you can edit your waterbrush and fringe it !
I found the exact same setting !
There is written ' border of Watercolor ' Click that.
Then you can edit the opacity ! If you play around with this you will make the fringe more visible, like this
So I hope this is helpful for some of you guys who uses this program <3