New Zealand, 2006 or so, and "yes". White or light colored jackets and helmets reduce incidence of multi-vehicle collision by 28%.
Not all motorcycles have large frontal areas. If you sit behind a huge white/yellow fairing, maybe the color of the jacket isn't as important, but the helmet might be. And with daytime running lights pretty much standard on cars now, the moto headlight doesn't stand out as much as it used to.
All my friends wear Aerostich RoadCrafters, hi-viz body with black ballistics (shoulders and forearm patches). Then they wear a traditional conspicuity vest (black with yellow reflective stripes) over the jacket- hiding most of the hi-viz.
I wear grey with yellow shoulders / forearms, sometimes a vest, sometimes not. Helmets are white (modular) or silver (full face). depending on if I'm riding across town or cross country.
Of course, the stats may be skewed- because if someone's going to think about safety enough to buy a white helmet over a black one, they may also take other aspects of riding safety more seriously.
77% of all moto crashes are the rider's fault, another 10% or so were avoidable by rider action.
As for riders choosing what [not] to wear when they go riding, I'm in the camp that thinks the ones who don't wear much, don't put much serious thought into it. I've been told "since I'm likely to die anyway, why wear all that junk?" which clearly discounts the thought that "all that junk" might reduce the level of injury. Not to mention, it can actually be more comfortable, if they get past the immediate perception, to gear up a little.
It's hard to properly place values on negative risk outcomes. This is a survival trait*, or we'd all still be in trees, throwing poop at the lions.
*As a species overall, not necessarily for the individual in particular
; that is, the survivors can learn from the mistakes made by the recently deceased.