Ashtrayhead said:
Whilst I agree that hi-viz stands out, it also seems that everyone and his dog is wearing it these days and it becomes so commonplace that it has the opposite effect of what it was intended.
In daylight there's a lot to be said for wearing a solid colour so that you will be silhouetted against the background of colours. (Having said that, a Metallica top is a slightly dubious choice!

)
Assuming you mean specifically a dark colour (after all, a hi-vis top can be a solid colour too!), I'm not sure that holds. A dark solid colour top stands out against a solid background of bright colour perhaps but in a scene as mixed as a street, one body sized object isn't going to stand out against all the other varying body sized colours - Hi vis still does stand out more, because even if a lot of people are wearing it, it shows against tarmac, buildings, white vans etc.
It may be commonplace, but it still stands out. Now, whether anyone
takes any notice of it, that's another thing. I work in hi-vis, alongside a bright yellow vehicle, often standing on the offside of it sorting recycling into the back, and most people give us plenty of room - there are still those who skim by at speed - it wouldn't matter if we were wearing hi-vis, or black, or stilts and stars and stripes top hats, they'd still be too lazy to slow down and turn their wheels a little bit to give us more room. Fortunately, we're all sensible and aware enough to look and listen and not step back unexpectedly....