Ive never seen your tecnique with priniting out just the black line but im doing one at the moment printing the whole color sections onto water slide decals which seems to be working ok but wont be anygood with light shining through it like yours

Printing the colours as well would make the job simpler, i thought of doing that myself, but decided not to go that way as i had the matching red paint on hand to use. Just in case you havent already done so, before placing water slide decal on the glass it might be a good idea to seal the glass with Triple Thick first, otherwise the water might lift the surrounding paint/artwork