if i were you, i would eliminate the outline in your stencil, letting the black background become the outline also. (that means you should even ignore the thin white line in my image below).
apart from that, you could cut out holes of all the green areas as one stencil for green
and cut out holes of all the other white areas as another stencil for silver
. unless that's not authentic. i'm only a moderate fan so i am not up on all the exactitudes of the potterverse.