In construction it is generally that only the banksman wears orange and everyone else wears yellow for distinction.

My father in law bought me a lime green parasol, it was covered in insects within an hour. I know that's not yellow or orange but I hope it helps as it is real evidence

Orange is the preferred colour for hi viz clothing on railways because yellow from a distant could be mistaken for a green signal, whereas seeing an orange signal would confer caution to a train driver. However, once when I was a train driver my view of operatives working trackside was obscured due to adjacent orange netlon fencing, so I reported it and it was changed to blue.

So the actual performance of hi viz clothing is partly dependant on the background colours. Construction plant and machinery are normally yellow/orange, which would be best yellow or orange hi viz clothing I could not say for sure - perhaps it should be blue? Hope that is objective enough for you.

Have a browse on Google Scholar.

I found this paper
http://74.125.155.132/sc...amp;hl=en&as_sdt=1,5

Yellow is brighter. If you look at Table 2 in EN471 which gives the colour requirements for "background material" you will see that the minimum luminance factor for yellow is almost twice that of orange-red.
The actual luminance of a material will vary with quality of the dye etc. Some time ago I attended a technical briefing at 3M's labs in Bracknell [I think] and the 3M presenter confirmed that a good yellow was significantly brighter than a good orange.

Of course brightness is only one factor in visibility... as the other posters have said contrast with the background is also very important. I seem to remember than when the RAC first changed their breakdown van livery to flourescent orange-pink they also issued their operatives with overalls in the same hi-vis colour before realising that the operatives 'disappeared' when stood next to their van. I believe it is also one of the reasons that the Highways Agency opted for yellow with red shoulders for their Traffic Officers so that they could still be seen when standing next to there yellow + black vehicles.

There are some opinions [can't find a reference] that two colour hi-vis also makes it easier to recognise the wearer as a 'thing' rather that just a block of colour.

Hope this helps