I'm not aware of any laws anywhere that require you to paint the tip of prop weapons.
However, many Cons will require it if you want to bring it into their Convention space, and many social media sites are very touchy about images showing weapons of any kind. I would wager that every content creator within the Halo costuming space has had to deal with photos/videos being taken down or highly restricted due to prop weapons. Orange tips don't inherently prevent that, but if it's a case where the image is flagged automatically by the algorithm but the appeal is handled by a real person, you have a better chance of winning with an orange tip versus without.
