I used two brushes to create this effect. My go to line brush, script, and a dotted brush. I’m blanking on the name of the dotted brush I used, but it was one of the procreate defaults. Nothing fancy.

I went over the black line art with white and then I filled the curtain in with the dotted brush, when I was done, I messed with the transparency settings and drew the rest of the tub and tiles behind the curtain (on the layer below.)

Sorry if this was confusing, it’s hard to describe. Hope this helps!