I am squarely in the bright colours camp. Most drivers will see you no matter what colour you and your panniers are but there are always a few inattentive drivers out there. Anything you can do to make myself more visible to them will increase your chances on the road.
In cases where there isn’t much room and a car or truck will come close to you, being brighter will make it easier for them to judge just how wide you are and miss you.
I see it as a numbers game. Out there some somewhere sometime is that unlucky combination of inattentive driver, narrow road, poor visibility and maybe even inattentive rider. Being brightly coloured is just trying to reduce the odds.
That said actually getting bright coloured panniers is not so easy. Even the yellow Ortleib panniers are mostly black when viewed from behind. I ended up asking Arkel (
www.arkel-od.com) to make me a special order of their GT-54 in their yellow fabric. I had to wait until they were making another batch to get them but I am happy with the result. I also have a yellow Ortleib rack pack that goes on top of my panniers, it stands out well.
Peter