Bright orange/red, solid. That will stand out best against a variety of backgrounds. White is good for night, but won't stand out against a bright sky (see the Tesla accident).

Patterns are a bad idea. People's brain automatically discard clutter, so from a distance you won't be visible no matter how bright the pattern. The warning cone pattern looks good from 30ft, but at 200ft I'm guessing it's less visible that any solid helmet.