Being the touring forum, I assume this is daytime and not night time.

I drove a motorcycle for several decades and from that it was my opinion that the most important days to worry about visibility were days with overcast and not enough sun to create shadows. Those were the days that I had the most close calls with car drivers acting as if I was invisible.

I think that yellowish green high visibility is best. Reflective does not help much if the car driver lights are off, but some people drive with lights on all the time or in overcast so it certainly can't hurt. And if a car was going to pull out in front of you from the side, their headlights would not be aimed in a way to make any reflective material accomplish anything.

I have a couple of windbreaker jackets that have zip off sleeves that I can convert to vests that are that greenish yellow. They are somewhat fadded but still is pretty good. And touring I usually wear jerseys of that color. In this example, that color seems to work well.




If you take a tour with Adventure Cycling Assoc., they issue you a triangle and mandate you have that on your bike or body. I have a couple of those and have decided to use those even when I am not riding with that group. On the trip from the photo below, one day I put the triangle on my left pannier instead of centered on rack top bag and it might be my imagination, but I think that car drivers started to give me more room when that triangle was on my left side. From now on I put that on the left pannier. (If I toured in UK, on the right side instead.)



The taillights in the photo above are pretty dim in the photo, but I usually have one on in flash mode, the other as a spare. But in rain and overcast, usually have both on in flash mode. But night time I have one or both on in constant on mode. They are pretty bright after a week of use, but I charge up the batteries after a week to make sure they stay bright.

I am not sure why the lights were so much brighter in the photo below, but maybe I got lucky and shot the photo just as both of them flashed?



I got my bright yellow rain jacket exchanged on warranty, I was pretty bummed that the most visible choice I had for a replacement was red, which is what I got. But on a rainy day, my taillights will be more important than my jacket color. And in this photo red does not look that bad.



I have considered sewing reflective stuff on my windbreakers, bought some reflective material. But have not gotten around to it yet. I bought the reflective material so that I could make my windbreakers conform with RUSA requirements for night riding on a brevet. I list their requirements here if you are interested.
https://rusa.org/reflectivity

Your question, vest or jacket or harness, what is best? Touring I have high vis jerseys and a windbreaker jacket that can convert to vest with zip off sleeves. All with that greenish yellow color.

Downsides? You have to buy it if you do not yet own it. And it makes it more obvious when you have done laundry.