All good answers so far, but, as it happens, although I was able to get a fluorescent green/yellow Giro Register MIPS helmet for my wife, that model, nor any other in the XL size I need only come in Black or White. I recall reading that Matte or Gloss White are both at the top of the visibility spectrum even when the other HI-Viz colors are considered. So my riding jacket is fluorescent green/yellow but my helmet is Matte White. The actual lifesaving, however, is done by superior road-craft and 50 years of experience in urban and suburban street riding. Proof? Well ... for forty of those years I wasn't riding with any kind of helmet, hi-viz, or even mirrors on the very same roads. So why change? I got tired of being pulled over by LEO for not having bells or lights or whatnot and the riding club we joined insisted on a helmet, so there you are. FWIW.