Riso style effect in photoshop

I'd like to start trying some riso style effects in photoshop - 

I have an already created artwork, is there a way to add some kind of effect that mimics the riso texture quite realistically? I have seen some tutorials for blending coloured layers together with the multiply efefct to mimic riso, but this is an already created artwork, so not sure if there's a way to separate different colours in an artwork out.

Can anyone also recommend some good riso brushes? 🙂