How accessible is the panel? And how large?
Would it be possible to tape some freezer paper over the whole window, and then draw round the colour shapes? You could then cut out the colours using a craft knife, leaving the background intact. Iron the background paper to your background fabric. The holes in the template would let you place the colours exactly, and then aplique using your favoured technique.
This would give a finished product the same size as your window, if that's what you want.....