To Nick Mahon
The current standards for GD(P) and GD(N) are CP2.
CP2 is normal colour vision.
Normal colour vision is getting 100% correct Ishihara plates (in daylight or daylight corrected light)
If you fail Ishihara and pass the Holmes-Wright lantern test you are CP3 ("colour-blind safe") and can hold an airfield driving permit for instance (traffic lights on airfield are not red amber green up a pole!)
Fail the H-W lantern you are CP4.