I was watching a video of a couple of touring riders. Recording was from behind and they had black panniers. You could not see pedaling motion, so nothing but black shape moving up a hill, on a curve, on what looked like a high speed roadway. Funny that some panniers have bright colors on the sides, but black front and back.

I usually put things in their own waterproof bags (those things that I want to stay dry like my sleeping bag). With fully waterproof panniers or like my waterproof backpack, the first morning when you stuff in your dew-wet or rain-wet stuff everything in there gets wet and the bag stays wet for days.