I think it looks elegant the way it is. If you MUST stitch the shapes, I think I would go with the color each one is and just straight stitch them on rather than a blanket stitch. A blanket stitch might look too 'homespun' unless that is the look you're going for. I happen to love blanket stitch, but a banner for a city might need to be more professional. The green may be a bit too light, I'm not sure. I stood about 10 feet away from my computer screen to see. It was visible, but it was a bit light. I like the idea someone had of putting fabric on top, but I wonder if you could peel the green off and lay underneath the green a darker fabric that would just outline the wonderful green you have there. If it were on black, no problem seeing it, but since is it laying on your yellow spotlight, that is your only issue. It may not be a problem, either.
Another thought is to outline the green in a dark green or black thread. I hope you win. Let us know what happens.