Their proper name is "Horn Lights" and I've been running a 4-LED green bulb with good success for about 3 years. They're very bright with a distinct, vibrant green glow that gets heads turning in traffic. Fortunately no attention from any law enforcement!

In any case, there's a tremendous variety of inexpensive multi-LED lights that you can try to find what suits you. I got mine from eBay for all of about $9 shipped, and that was 3 years ago.