The RSSB has a large number of studies and research projects that are able to be viewed if you have a suitable login/membership.
Colour vision testing :
https://www.rssb.co.uk/research-catalogue/CatalogueItem/S352
I can't seem to get an individual registration to RSSB to work but does it quantify what level of deficiency might be appropriate or talk about what testing method could be used to find the acceptable level of deficiency decided on?
Perhaps to be a bit clearer than I was originally the CAA spent time and money establishing a single standard for all pilots in the UK along with methods that testing from other aviation authorities could be equated to that. However, that was only for effectively a single role with the approximate equivalency to that of a driver. The requirement to see and interpret coloured light signals could be argued to be different between driver, guard and dispatcher (i.e. stationary in a platform and not at line speed so speed of interpretation is different or arguably only needing to see that the signal is showing a proceed aspect so less requirement to see the red part of the spectrum etc.) Each role should arguably have a level of deficiency allowable along with a rationale for why that is the case, this is the time and research cost that I was referring to.
Now it would be relatively easy to establish a new single standard as exists currently but would it actually be a change to the current situation or just a new blanket standard with a different testing rationale?
I completely get your point that some people want and indeed have requested change but maybe I'm too cynical that absent the commercial pressure from training providers, who make money training private and commercial pilots, that drove the CAA to change I can't see ORR wanting to.