Walk into a workwear shop and you'll see a range of colours for hi-vis clothing. Orange, yellow, green, and chartreuse all compete for your attention on the clothing racks. The big question is, which of those colours will get drivers' attention when you're actually at work? All of them can, but expectations and the surrounding environment affect how well those colours show up.
Day and Night Visibility
If you've got crews working on projects day and night, you'll need different colours. Despite the bright lights used on nighttime construction and other projects, orange isn't the most visible colour. You may still see orange cones and pylons because they're in line with car headlights, but workwear that's orange will not necessarily be noticeable, especially if the worker has to move out of the spotlight created by the construction lights. Yellow and chartreuse would provide more contrast at night. Conversely, during the day, orange would be very noticeable because yellow and chartreuse (and even light green) can seem more washed out in daylight.
Almost Universal Associations
When people see someone walking around in an orange jacket, they instinctively know to take notice. That's because orange has become associated with caution. Hunting gear includes orange visibility clothing to ensure you aren't mistaken for a deer among dense trees, for example. People see those orange traffic cones and look for construction. Even road flares are orange or red. If you're in a situation where orange stands out, that can be a great colour to use.
You also want to be sure the colours stand out against backgrounds during the day. Chartreuse and yellow shirts, for example, aren't as visible against light green lawns or against buildings painted yellow. You don't see as many orange buildings, so orange is useful if the area in which you'll work has a lot of green and yellow in the background. But if you work in an area where the buildings or roads are darker colours, then a set of yellow workwear could be very visible because of the higher contrast. This does not mean that orange is the only colour you should use at all. It means you have to evaluate when and where you'll be working and have the best colour available. In an ideal world, you'd have workwear in multiple colours that you could change depending on the circumstances, so you may want to ask about bulk purchasing for your crew.