Paint how to make it with alpha

So I've used multistroke to clean background, now I'd love to add grain on only those areas, how could I separate what's been painted olny?
are mutistrokes and strokes in the modifier could be copied say to a mask paint?

Difference keyer usually works fine for this purpose and is faster than anything else.

Thanks! I've made already with Bol, but will try this one too!