Let me start by saying I have no problem with cyclists using roads. I give them space, I hang back, I only ever overtake when it is definitely safe to do so despite this often enraging the driver behind me.
BUT.
I live really rurally. The main road through my village is a very popular one with cyclists as it is a 20 mile trip through a beautiful river valley. It's twisty, its turny, it's uppy downy. It also passes in and out of thick woodland along the whole route. So visibility is crap full stop.
A worrying majority of the cyclists I see on this road are wearing black and flouro yellow. This means they show up beautifully on the few open sections of road where it passes through villages and would therefore be great in urban areas, but makes them almost completely invisible when going in and out of tree shade or along tree lined roads. The black and yellow just blend in perfectly with the dappled shade through leaves on tarmac. You just can't see them until you're almost on top of them because the hi vis they've chosen isn't hi vis at all in the conditions they're actually riding in! It's practically camouflage.
Sometimes they have lights on, but all too often if it's daylight they don't. One I only just saw last week was in black and yellow with a black rucksack, nothing reflective, but a red flashing light on a rucksack strap that had fallen to the side so wasn't visible until you were to the side of him! Reflective strips are more common but they're not that much use when it's light as cars often don't have headlights on in the daytime (my current car does because it's got running lights, but that's not standard).
Every now and then I see a cyclist using bright pink, orange, or blue, and they stand out so well! You can spot them even in the mix of sun and shade that makes the black and yellow ones almost invisible.
It's so common for the "hi vis" they wear to be black with yellow bits that I assume it just isn't known about? Can any cyclists tell me if they know this?