Always had a bit of curiosity on this, but never done any investigation. I was always told you either are or you are not color blind. It just does not happen overnight. I hate those colored numbers, some are hard and some doctors just wait for you to say..Doc I cant see any numbers, then he says good, thats a test page...
In the States regs are limited unless you talk to an FAA Flight Surgeon.
heres what I found for the Average Pilots info..
Part 67.103 (c), 67.203 (c), 67.303 (c)
Ability to perceive those colors necessary for the safe performance of airman duties.
That covers all three classes of licenses.
Not really much help. Wish there was a Flight Surgeon who could jump in.